alter table新增欄位操作究竟有何影響？(下篇)
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有點扯遠了，說正事兒，作為本篇博文的內容，主要包含兩個方面：
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1.<strong style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">話題1：不同鎖模式的實驗</strong>
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接著上篇博文的話題，針對幾種常見的鎖模式透過實驗感受下之間的不同。
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2.<strong style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">話題2：不同方式新增欄位的效率</strong>
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最近有個系統執行新增欄位的操作出現了hang，藉此使用實驗說明下不同新增欄位的方式對效率的影響和原理。<br />
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話題1：不同鎖模式的實驗
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上篇博文檢視了執行如下新增欄位的trace檔案：<span class="Apple-converted-space"> </span><code style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:1px;border-right-width:1px;border-bottom-width:1px;border-left-width:1px;border-style:initial;border-color:initial;font-size:12px;font-family:Consolas, 'Liberation Mono', Courier, monospace;white-space:nowrap;border-top-style:solid;border-right-style:solid;border-bottom-style:solid;border-left-style:solid;border-top-color:#EAEAEA;border-right-color:#EAEAEA;border-bottom-color:#EAEAEA;border-left-color:#EAEAEA;background-color:#F8F8F8;border-top-left-radius:3px 3px;border-top-right-radius:3px 3px;border-bottom-right-radius:3px 3px;border-bottom-left-radius:3px 3px;">alter table t add (sex varchar2(1));</code>
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發現執行該語句時是以NOWAIT方式對錶新增了一個ROW EXCLUSIVE模式鎖：<code style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:1px;border-right-width:1px;border-bottom-width:1px;border-left-width:1px;border-style:initial;border-color:initial;font-size:12px;font-family:Consolas, 'Liberation Mono', Courier, monospace;white-space:nowrap;border-top-style:solid;border-right-style:solid;border-bottom-style:solid;border-left-style:solid;border-top-color:#EAEAEA;border-right-color:#EAEAEA;border-bottom-color:#EAEAEA;border-left-color:#EAEAEA;background-color:#F8F8F8;border-top-left-radius:3px 3px;border-top-right-radius:3px 3px;border-bottom-right-radius:3px 3px;border-bottom-left-radius:3px 3px;">LOCK TABLE "T" IN ROW EXCLUSIVE MODE NOWAIT</code>
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我們看下Oracle的官方解釋：
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ROW SHARE
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ROW SHARE permits concurrent access to the locked table but prohibits users from locking the entire table for exclusive access.
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<strong style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">ROW SHARE允許併發訪問被鎖定的表，但是禁止使用者以排他訪問的方式鎖定整張表</strong>。
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ROW EXCLUSIVE
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ROW EXCLUSIVE is the same as ROW SHARE, but it also prohibits locking in SHARE mode. ROW EXCLUSIVE locks are automatically obtained when updating, inserting, or deleting.
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<strong style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">ROW EXCLUSIVE和ROW SHARE相同，但禁止以SHARE模式鎖定。當執行update、insert或delete語句時會自動獲得ROW EXCLUSIVE鎖</strong>。
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我是初學者，反正我是沒太明白兩者的區別，晦澀。只有透過實驗，才是最有助於理解其含義的方法。
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```
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實驗版本：
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SQL> select * from v$version where rownum = 1;
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Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
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實驗1：
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session 1執行：
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SQL> lock table t in row share mode;
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Table(s) Locked.
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session 2此時可以執行以下語句：
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SQL> update t set a = 1 where id =1;
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1 row updated.
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SQL> select count(*) from t;
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1000000
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實驗2：
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session 1和session 2都可以執行：
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SQL> lock table t in row share mode;
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Table(s) Locked.
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此時session 1可以執行：
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update t set a = 1 where id = 1;
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1 row updated.
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此時session 2執行以下語句會hang：
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update t set a = 1 where id = 1;
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但可以對其他行記錄操作：
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SQL> update t set b = 'B' where id = 2;
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1 row updated.
</p>
<hr style="clear:both;margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;height:0px;overflow-x:hidden;overflow-y:hidden;border-top-style:none;border-right-style:none;border-bottom-style:solid;border-left-style:none;border-width:initial;border-color:initial;background-image:initial;background-attachment:initial;background-origin:initial;background-clip:initial;background-color:transparent;border-bottom-width:4px;border-bottom-color:#DDDDDD;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;background-position:initial initial;background-repeat:initial initial;" />
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗3：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session 2可以執行以下兩條語句：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
但session 2執行以下語句hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
從ROW SHARE的效果來看，這種行級鎖，允許不同session同時持有ROW SHARE或SHARE或ROW EXCLUSIVE鎖，但某一session執行DML語句後，其他session就無法針對相同的資料行做DML操作，處於hang，除非上一session的DML操作commit或rollback，但此時還是可以允許併發的只讀訪問。但不允許其他session獲得EXCLUSIVE鎖。證明了Oracle官方所說的“禁止使用者以排他訪問的方式鎖定整張表”。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
接下來看看ROW EXCLUSIVE模式鎖的實驗。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗1：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 2以下語句均可執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> select count(*) from t;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
1000000
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> update t set a = 1 where id = 1;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
1 row updated.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
但此時session 1再執行同行的操作則會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> update t set a = 1 where id = 1;
</p>
<hr style="clear:both;margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;height:0px;overflow-x:hidden;overflow-y:hidden;border-top-style:none;border-right-style:none;border-bottom-style:solid;border-left-style:none;border-width:initial;border-color:initial;background-image:initial;background-attachment:initial;background-origin:initial;background-clip:initial;background-color:transparent;border-bottom-width:4px;border-bottom-color:#DDDDDD;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;background-position:initial initial;background-repeat:initial initial;" />
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗2：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1和session 2均可執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
如果session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> update t set a = 1 where id = 1;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
1 row updated.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session 2執行以下語句就會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> update t set a = 1 where id = 1;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1和session 2均可執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> select count(*) from t;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
1000000
</p>
<hr style="clear:both;margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;height:0px;overflow-x:hidden;overflow-y:hidden;border-top-style:none;border-right-style:none;border-bottom-style:solid;border-left-style:none;border-width:initial;border-color:initial;background-image:initial;background-attachment:initial;background-origin:initial;background-clip:initial;background-color:transparent;border-bottom-width:4px;border-bottom-color:#DDDDDD;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;background-position:initial initial;background-repeat:initial initial;" />
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗3：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session 2可以執行以下兩條語句：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
但執行以下兩條語句會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
從ROW EXCLUSIVE的效果來看，這種行級鎖，允許不同session同時持有ROW EXCLUSIVE或ROW SHARE鎖，但某一session執行DML語句後，其他session就無法針對相同的資料行做DML操作，處於hang，除非上一session的DML操作commit或rollback，但此時還是可以允許併發的只讀訪問。即他允許多個會話擁有ROW行級EXCLUSIVE或SHARE鎖，但無法同時獲得EXCLUSIVE或SHARE鎖，從限制上要比ROW SHARE更嚴格。證明了Oracle官方所說的“禁止以SHARE模式鎖定”，EXCLUSIVE比SHARE更嚴格，自然也不能獲得EXCLUSIVE鎖。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
以上是對兩種ROW行級鎖的實驗，結論就是ROW EXCLUSIVE和ROW SHARE均可以允許併發只讀操作，從鎖的強弱看，ROW EXCLUSIVE > ROW SHARE，但其實這種行級鎖可能更多地還是透過DML語句自動獲得，而不是用實驗中的LOCK語句。以上只是為了更好地說明兩者區別。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
說完了行級鎖，接下來看下錶級鎖：
</p>
<blockquote style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:15px;padding-bottom:0px;padding-left:15px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:4px;border-style:initial;border-color:initial;border-left-style:solid;border-left-color:#DDDDDD;color:#777777;">
<p style="margin-top:0px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SHARE
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SHARE permits concurrent queries but prohibits updates to the locked table.
</p>
</blockquote>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<strong style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">SHARE鎖允許併發查詢，但是禁止其他session對鎖定的表更新。</strong>
</p>
<blockquote style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:15px;padding-bottom:0px;padding-left:15px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:4px;border-style:initial;border-color:initial;border-left-style:solid;border-left-color:#DDDDDD;color:#777777;">
<p style="margin-top:0px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
EXCLUSIVE
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
EXCLUSIVE permits queries on the locked table but prohibits any other activity on it.
</p>
</blockquote>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<strong style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">EXCLUSIVE允許鎖定表的查詢操作，但禁止其他session對該表的任何操作</strong>
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
我覺得這裡Oracle的介紹是有些問題的，不夠嚴謹，至少沒有說清楚到底限制有何不同，接下來，我們還是透過實驗的方式進行說明。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
首先來看SHARE模式鎖：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗1：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session 2可以執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> select count(*) from t;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
1000000
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
但執行以下語句會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> update t set b = 'a' where a = 1;
</p>
<hr style="clear:both;margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;height:0px;overflow-x:hidden;overflow-y:hidden;border-top-style:none;border-right-style:none;border-bottom-style:solid;border-left-style:none;border-width:initial;border-color:initial;background-image:initial;background-attachment:initial;background-origin:initial;background-clip:initial;background-color:transparent;border-bottom-width:4px;border-bottom-color:#DDDDDD;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;background-position:initial initial;background-repeat:initial initial;" />
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗2：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1和session 2均可執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1和session 2均可執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> select count(*) from t;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
1000000
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
但此時無論是session 1還是session 2先執行以下語句都會hang(比如此處是session 1先執行)：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> update t set b = 'a' where a = 1;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session 2執行以下語句也會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> update t set b = 'a' where a = 1;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
但同時session 1處於hang的語句會報錯：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> update t set b = 'a' where a = 1;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
update t set b = 'a' where a = 1
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
*
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
ERROR at line 1:
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
ORA-00060: deadlock detected while waiting for resource
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session2還處於hang的狀態。
</p>
<hr style="clear:both;margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;height:0px;overflow-x:hidden;overflow-y:hidden;border-top-style:none;border-right-style:none;border-bottom-style:solid;border-left-style:none;border-width:initial;border-color:initial;background-image:initial;background-attachment:initial;background-origin:initial;background-clip:initial;background-color:transparent;border-bottom-width:4px;border-bottom-color:#DDDDDD;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;background-position:initial initial;background-repeat:initial initial;" />
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗3：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session 2可以執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
但以下兩條語句均會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
從SHARE模式鎖的效果來看，
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
(1) 如果某一session獲得SHARE模式鎖後，其他session還可以執行DML操作。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
(2) 如果多個session同時獲得SHARE模式鎖，則這些session只能執行讀操作，做DML操作會hang。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
(3) 如果兩個session同時對一行記錄做DML操作，則第一個session會報ORA-60死鎖錯誤，直接被Oracle檢測退出，第二個session繼續處於hang。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
(4) 如果一個session獲得了SHARE模式鎖，則其他session不能再獲得ROW EXCLUSIVE或EXCLUSIVE模式鎖，但可以獲得ROW SHARE或SHARE模式鎖。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
因此對開始的介紹：
</p>
<blockquote style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:15px;padding-bottom:0px;padding-left:15px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:4px;border-style:initial;border-color:initial;border-left-style:solid;border-left-color:#DDDDDD;color:#777777;">
<p style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SHARE鎖允許併發查詢，但是禁止其他session對鎖定的表更新。
</p>
</blockquote>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
更嚴謹的是說對多個獲得SHARE鎖的session來說，允許併發讀，但禁止做DML操作，即只需看，不許改，這也是SHARE的含義。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
接下來看看EXCLUSIVE模式鎖的實驗。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗1：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session 2可以執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> select count(*) from t;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
1000000
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
但執行以下語句會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> update t set b = 'a' where a = 1;
</p>
<hr style="clear:both;margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;height:0px;overflow-x:hidden;overflow-y:hidden;border-top-style:none;border-right-style:none;border-bottom-style:solid;border-left-style:none;border-width:initial;border-color:initial;background-image:initial;background-attachment:initial;background-origin:initial;background-clip:initial;background-color:transparent;border-bottom-width:4px;border-bottom-color:#DDDDDD;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;background-position:initial initial;background-repeat:initial initial;" />
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗2：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 2執行以下語句會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in exclusive mode;
</p>
<hr style="clear:both;margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;height:0px;overflow-x:hidden;overflow-y:hidden;border-top-style:none;border-right-style:none;border-bottom-style:solid;border-left-style:none;border-width:initial;border-color:initial;background-image:initial;background-attachment:initial;background-origin:initial;background-clip:initial;background-color:transparent;border-bottom-width:4px;border-bottom-color:#DDDDDD;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;background-position:initial initial;background-repeat:initial initial;" />
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗3：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session 2執行以下語句均會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
``` 從EXCLUSIVE模式鎖的效果來看，
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
(1) 如果某一session獲得EXCLUSIVE模式鎖，則其他session只能允許讀操作，禁止DML操作。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
(2) 如果某一session獲得EXCLUSIVE模式鎖，則禁止其他session再獲得ROW SHARE、SHARE、ROW EXCLUSIVE或EXCLUSIVE各種模式鎖。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
因此對開始的介紹：
</p>
<blockquote style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:15px;padding-bottom:0px;padding-left:15px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:4px;border-style:initial;border-color:initial;border-left-style:solid;border-left-color:#DDDDDD;color:#777777;">
<p style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
EXCLUSIVE允許鎖定表的查詢操作，但禁止其他session對該表的任何操作。
</p>
</blockquote>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
更嚴謹的是說對多個蝴蝶EXCLUSIVE模式鎖的session來說，除了讀操作外，禁止其他任何操作。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
顯然，從鎖的強弱看，EXCLUSIVE>SHARE>ROW EXCLUSIVE>ROW SHARE。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
另外，還有一種鎖
</p>
<blockquote style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:15px;padding-bottom:0px;padding-left:15px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:4px;border-style:initial;border-color:initial;border-left-style:solid;border-left-color:#DDDDDD;color:#777777;">
<p style="margin-top:0px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SHARE ROW EXCLUSIVE
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SHARE ROW EXCLUSIVE is used to look at a whole table and to allow others to look at rows in the table but to prohibit others from locking the table in SHARE mode or from updating rows.
</p>
</blockquote>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
** SHARE ROW EXCLUSIVE模式鎖用來檢視整張表，允許其他session檢索表中的行，但禁止其他session以SHARE模式鎖定表或者更新行。 **
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗1：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session 2可以執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> select count(*) from t;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
1000000
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
但執行以下語句會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> update t set b = 'a' where a = 1;
</p>
<hr style="clear:both;margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;height:0px;overflow-x:hidden;overflow-y:hidden;border-top-style:none;border-right-style:none;border-bottom-style:solid;border-left-style:none;border-width:initial;border-color:initial;background-image:initial;background-attachment:initial;background-origin:initial;background-clip:initial;background-color:transparent;border-bottom-width:4px;border-bottom-color:#DDDDDD;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;background-position:initial initial;background-repeat:initial initial;" />
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗2：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 2執行以下語句會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share row exclusive mode;
</p>
<hr style="clear:both;margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;height:0px;overflow-x:hidden;overflow-y:hidden;border-top-style:none;border-right-style:none;border-bottom-style:solid;border-left-style:none;border-width:initial;border-color:initial;background-image:initial;background-attachment:initial;background-origin:initial;background-clip:initial;background-color:transparent;border-bottom-width:4px;border-bottom-color:#DDDDDD;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;background-position:initial initial;background-repeat:initial initial;" />
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗3：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
session 1執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table(s) Locked.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
此時session 2可以執行：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
但session 2執行以下語句均會hang：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in share mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in row exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> lock table t in exclusive mode;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
從SHARE ROW EXCLUSIVE的效果來看，相比SHARE允許其他session同時獲得SHARE模式鎖，其禁止其他session獲得SHARE模式鎖。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
從鎖的強弱看，EXCLUSIVE(exclusive，X)>SHARE ROW EXCLUSIVE(S/Row-X，SRX)>SHARE(Share，S)>ROW EXCLUSIVE(Row-X，RX)>ROW SHARE(Row-S，RS)。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
引述一篇部落格的總結(http://blog.itpub.net/9252210/viewspace-626388/)
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
2級鎖Row-S 行共享(RS)：共享表鎖，sub share，鎖有：Select for update,Lock For Update,Lock Row Share。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
3級鎖Row-X 行獨佔(RX)：用於行的修改，sub exclusive，鎖有：Insert, Update, Delete, Lock Row Exclusive。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
4級鎖Share 共享鎖(S)：阻止其他DML操作，share，鎖有：Create Index, Lock Share，locked_mode為2，3，4不影響DML(insert,delete,update,select)操作, 但DDL(alter,drop等)操作會hang。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
5級鎖S/Row-X 共享行獨佔(SRX)：阻止其他事務操作，share/sub exclusive，鎖有：Lock Share Row Exclusive，具體來講有主外來鍵約束時update / delete ... ; 可能會產生4，5的鎖。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
6級鎖exclusive 獨佔(X)：獨立訪問使用，exclusive，鎖有：Alter table, Drop table, Drop Index, Truncate table, Lock Exclusive。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
數字越大鎖級別越高, 影響的操作越多。
</p>
<h2 style="margin-top:20px;margin-right:0px;margin-bottom:10px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:1px;border-left-width:0px;border-style:initial;border-color:initial;font-weight:bold;-webkit-font-smoothing:antialiased;font-size:24px;border-bottom-style:solid;border-bottom-color:#CCCCCC;color:#000000;">
話題2：不同方式新增欄位的效率
</h2>
<p style="margin-top:10px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗1：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> set timing on
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> alter table t add add_a number;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table altered.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Elapsed: 00:00:00.29
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
新增一個允許NULL，且無預設值的欄位，用時0.29秒。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
前文介紹了，獲得的是一個ROW EXCLUSIVE模式鎖。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<code style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:1px;border-right-width:1px;border-bottom-width:1px;border-left-width:1px;border-style:initial;border-color:initial;font-size:12px;font-family:Consolas, 'Liberation Mono', Courier, monospace;white-space:nowrap;border-top-style:solid;border-right-style:solid;border-bottom-style:solid;border-left-style:solid;border-top-color:#EAEAEA;border-right-color:#EAEAEA;border-bottom-color:#EAEAEA;border-left-color:#EAEAEA;background-color:#F8F8F8;border-top-left-radius:3px 3px;border-top-right-radius:3px 3px;border-bottom-right-radius:3px 3px;border-bottom-left-radius:3px 3px;">LOCK TABLE "T" IN ROW EXCLUSIVE MODE NOWAIT</code>
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗2：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> alter table t add add_b number default 0;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table altered.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Elapsed: 00:00:59.34
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
新增一個允許NULL，但有預設值的欄位，用時59秒。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
從10046的trace檔案看，他會首先用EXCLUSIVE模式鎖來鎖定表。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<code style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:1px;border-right-width:1px;border-bottom-width:1px;border-left-width:1px;border-style:initial;border-color:initial;font-size:12px;font-family:Consolas, 'Liberation Mono', Courier, monospace;white-space:nowrap;border-top-style:solid;border-right-style:solid;border-bottom-style:solid;border-left-style:solid;border-top-color:#EAEAEA;border-right-color:#EAEAEA;border-bottom-color:#EAEAEA;border-left-color:#EAEAEA;background-color:#F8F8F8;border-top-left-radius:3px 3px;border-top-right-radius:3px 3px;border-bottom-right-radius:3px 3px;border-bottom-left-radius:3px 3px;">LOCK TABLE "T" IN EXCLUSIVE MODE NOWAIT</code>
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
同時，在最後執行了更新欄位ADD_B為預設值的操作：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<code style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:1px;border-right-width:1px;border-bottom-width:1px;border-left-width:1px;border-style:initial;border-color:initial;font-size:12px;font-family:Consolas, 'Liberation Mono', Courier, monospace;white-space:nowrap;border-top-style:solid;border-right-style:solid;border-bottom-style:solid;border-left-style:solid;border-top-color:#EAEAEA;border-right-color:#EAEAEA;border-bottom-color:#EAEAEA;border-left-color:#EAEAEA;background-color:#F8F8F8;border-top-left-radius:3px 3px;border-top-right-radius:3px 3px;border-bottom-right-radius:3px 3px;border-bottom-left-radius:3px 3px;">update "T" set "ADD_B"=0;</code>
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
因此不難想為什麼耗時這樣久，就是還需要更新所有欄位為預設值，另外，還會因為資料量的增加，可能需要更多的UNDO空間，進而可能因為一條新增欄位的操作，導致整個庫的UNDO表空間不夠用，不僅影響對這張表的正常增刪改操作(因為獲取了最高階別EXCLUSIVE鎖)，還有可能影響其他業務功能(因為UNDO表空間不夠用)。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
實驗3：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> alter table t add add_c number default 0 not null;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table altered.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Elapsed: 00:00:00.16 ```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
新增一個包含NOT NULL約束，有預設值的欄位，用時0.16秒。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
從10046的trace檔案看，會獲得一個ROW EXCLUSIVE模式鎖來鎖定表。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<code style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:1px;border-right-width:1px;border-bottom-width:1px;border-left-width:1px;border-style:initial;border-color:initial;font-size:12px;font-family:Consolas, 'Liberation Mono', Courier, monospace;white-space:nowrap;border-top-style:solid;border-right-style:solid;border-bottom-style:solid;border-left-style:solid;border-top-color:#EAEAEA;border-right-color:#EAEAEA;border-bottom-color:#EAEAEA;border-left-color:#EAEAEA;background-color:#F8F8F8;border-top-left-radius:3px 3px;border-top-right-radius:3px 3px;border-bottom-right-radius:3px 3px;border-bottom-left-radius:3px 3px;">LOCK TABLE "T" IN ROW EXCLUSIVE MODE NOWAIT</code>`
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
鎖的級別比實驗2要低，而且該預設值是儲存於col$資料字典表中的，並不是儲存在原表記錄上，這點的原因可以參見David的博文(http://blog.csdn.net/tianlesoftware/article/details/7226893)。即新增一個NOT NULL和預設值的欄位，以後每次需要使用該欄位時，預設值都是從資料字典中查詢到的，這樣就減少了新增欄位時的DDL語句時間，也減少了儲存空間(不用每條需要使用預設值的記錄都儲存預設值)。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
``` 實驗4：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> select count(*) from t;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
1000000
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> alter table t add add_h number not null;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
alter table t add add_h number not null
</p>
<pre style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:6px;padding-right:10px;padding-bottom:6px;padding-left:10px;border-top-width:1px;border-right-width:1px;border-bottom-width:1px;border-left-width:1px;border-style:initial;border-color:initial;font-size:13px;font-family:Consolas, 'Liberation Mono', Courier, monospace;background-color:#F8F8F8;border-top-style:solid;border-right-style:solid;border-bottom-style:solid;border-left-style:solid;border-top-color:#CCCCCC;border-right-color:#CCCCCC;border-bottom-color:#CCCCCC;border-left-color:#CCCCCC;line-height:19px;overflow-x:auto;overflow-y:auto;border-top-left-radius:3px 3px;border-top-right-radius:3px 3px;border-bottom-right-radius:3px 3px;border-bottom-left-radius:3px 3px;"><code style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-style:initial;border-color:initial;font-size:12px;font-family:Consolas, 'Liberation Mono', Courier, monospace;white-space:pre;border-top-style:none;border-right-style:none;border-bottom-style:none;border-left-style:none;background-color:transparent;border-top-left-radius:3px 3px;border-top-right-radius:3px 3px;border-bottom-right-radius:3px 3px;border-bottom-left-radius:3px 3px;border-width:initial;background-image:initial;background-attachment:initial;background-origin:initial;background-clip:initial;background-position:initial initial;background-repeat:initial initial;"> * </code></pre>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
ERROR at line 1:
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
ORA-01758: table must be empty to add mandatory (NOT NULL) column
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
新增一個僅有NOT NULL約束，沒有預設值的欄位，則需要表為空。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
從11g之後，對於新增欄位，Oracle進行了最佳化，引述官方介紹：
</p>
<blockquote style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:15px;padding-bottom:0px;padding-left:15px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:4px;border-style:initial;border-color:initial;border-left-style:solid;border-left-color:#DDDDDD;color:#777777;">
<p style="margin-top:0px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Adding Table Columns
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
If a new column is added to a table, the column is initially NULL unless you specify the DEFAULT clause. When you specify a default value, the database immediately updates each row with the default value. Note that this can take some time, and that during the update, there is an exclusive DML lock on the table. For some types of tables (for example, tables without LOB columns), if you specify both a NOT NULL constraint and a default value, the database can optimize the column add operation and greatly reduce the amount of time that the table is locked for DML.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
You can add a column with a NOT NULL constraint only if the table does not contain any rows, or you specify a default value.
</p>
</blockquote>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
說的很明白了，如果新增一個含有預設值的欄位，那麼會立即更新每一行，在更新過程中，會有一個EXCLUSIVE級別的鎖在該表上。如果指定NOT NULL和預設值，則會進行最佳化，降低阻止DML操作的時間。如果增加一個僅有NOT NULL的約束欄位，那麼需要表不能包含任何記錄，否則就需要必須指定一個預設值，這也好理解，如果執行之前有記錄，又要求NOT NULL，那麼之前的記錄欄位預設值是什麼就需要指定才行。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
順帶提一句，刪除表欄位的操作：
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
SQL> alter table t drop column add_b;
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Table altered.
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
Elapsed: 00:00:43.44
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
```
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
從10046的trace檔案看，也是獲得了一個EXCLUSIVE鎖，進而更新的過程中是對整張表的DML操作有影響的。
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<code style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:1px;border-right-width:1px;border-bottom-width:1px;border-left-width:1px;border-style:initial;border-color:initial;font-size:12px;font-family:Consolas, 'Liberation Mono', Courier, monospace;white-space:nowrap;border-top-style:solid;border-right-style:solid;border-bottom-style:solid;border-left-style:solid;border-top-color:#EAEAEA;border-right-color:#EAEAEA;border-bottom-color:#EAEAEA;border-left-color:#EAEAEA;background-color:#F8F8F8;border-top-left-radius:3px 3px;border-top-right-radius:3px 3px;border-bottom-right-radius:3px 3px;border-bottom-left-radius:3px 3px;">LOCK TABLE "T" IN EXCLUSIVE MODE NOWAIT</code>
</p>
<p style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
總結起來：
</p>
<ol style="margin-top:15px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:30px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<li style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<p style="margin-top:0px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
11g以上的版本，如果使用NOT NULL和預設值的方式新增欄位，那麼執行時間會大大降低。且只會有一個ROW EXCLUSIVE級別鎖。
</p>
</li>
<li style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<p style="margin-top:0px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
11g以上的版本，如果使用預設值，沒有NOT NULL約束的方式新增欄位，那麼執行時間會很久，取決於表中資料量的大小，獲得的是EXCLUSIVE級別鎖，期間會影響所有記錄的DML操作，可能會因UNDO不足對其他操作有影響。
</p>
</li>
<li style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
<p style="margin-top:0px;margin-right:0px;margin-bottom:15px;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
11g以上的版本，如果新增欄位沒有預設值，也沒有NOT NULL約束，則還是會使用ROW EXCLUSIVE模式鎖，但由於不需要更新欄位值，執行時間也是比較短。
</p>
</li>
</ol>
<p style="margin-top:15px;margin-right:0px;margin-bottom:0px !important;margin-left:0px;padding-top:0px;padding-right:0px;padding-bottom:0px;padding-left:0px;border-top-width:0px;border-right-width:0px;border-bottom-width:0px;border-left-width:0px;border-style:initial;border-color:initial;">
如何選擇11g上新增欄位的方式，看來是有一個比較清晰的方向了。
</p>
</div>
</span>
來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/30496894/viewspace-1815866/，如需轉載，請註明出處，否則將追究法律責任。
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