個人部落格地址:www.chengxy-nds.top,技術資料自取
昨天中午在食堂,和部門的技術大牛們坐在一桌吃飯,作為一個卑微技術渣仔默默的吃著飯,聽大佬們高談闊論,研究各種高階技術,我TM也想說話可實在插不上嘴。
聊著聊著突然說到他上午面試了一個工作6年的程式設計師,表情挺複雜,他說:我看他簡歷寫著熟悉SQL
語句調優,就問了下 Explain
執行計劃怎麼看?結果這老哥一問三不知,工作6年這麼基礎的東西都不瞭解!
感受到了大佬的王之鄙視,回到工位我就開始默默寫這個,哎~ 我TM也不太懂 Explain
,老哥你這是針對我啊!哭唧唧~
Explain有什麼用
當Explain
與 SQL
語句一起使用時,MySQL
會顯示來自最佳化器關於SQL執行的資訊。也就是說,MySQL
解釋了它將如何處理該語句,包括如何連線表以及什麼順序連線表等。
Explain
執行計劃包含欄位資訊如下:分別是 id
、select_type
、table
、partitions
、type
、possible_keys
、key
、key_len
、ref
、rows
、filtered
、Extra
12個欄位。
下邊我們會結合具體的SQL
示例,詳細的解讀每個欄位以及每個欄位中不同引數的含義,以下所有示例資料庫版本為 MySQL.5.7.17
。
mysql> select version() from dual;
+------------+
| version() |
+------------+
| 5.7.17-log |
+------------+
我們建立三張表 one
、two
、three
,表之間的關係 one.two_id = two.two_id AND two.three_id = three.three_id
。
Explain執行計劃詳解
一、id
id:
:表示查詢中執行select子句或者操作表的順序,id
的值越大,代表優先順序越高,越先執行。 id
大致會出現 3種情況:
1、id
相同
看到三條記錄的id
都相同,可以理解成這三個表為一組,具有同樣的優先順序,執行順序由上而下,具體順序由最佳化器決定。
mysql> EXPLAIN SELECT * FROM one o,two t, three r WHERE o.two_id = t.two_id AND t.three_id = r.three_id;
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | o | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL |
| 1 | SIMPLE | t | NULL | ALL | PRIMARY | NULL | NULL | NULL | 2 | 50 | Using where; Using join buffer (Block Nested Loop) |
| 1 | SIMPLE | r | NULL | eq_ref | PRIMARY | PRIMARY | 4 | xin-slave.t.three_id | 1 | 100 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
2、id
不同
如果我們的 SQL
中存在子查詢,那麼 id
的序號會遞增,id
值越大優先順序越高,越先被執行 。當三個表依次巢狀,發現最裡層的子查詢 id
最大,最先執行。
mysql> EXPLAIN select * from one o where o.two_id = (select t.two_id from two t where t.three_id = (select r.three_id from three r where r.three_name='我是第三表2'));
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | o | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
| 2 | SUBQUERY | t | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
| 3 | SUBQUERY | r | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
3、以上兩種同時存在
將上邊的 SQL
稍微修改一下,增加一個子查詢,發現 id
的以上兩種同時存在。相同id
劃分為一組,這樣就有三個組,同組的從上往下順序執行,不同組 id
值越大,優先順序越高,越先執行。
mysql> EXPLAIN select * from one o where o.two_id = (select t.two_id from two t where t.three_id = (select r.three_id from three r where r.three_name='我是第三表2')) AND o.one_id in(select one_id from one where o.one_name="我是第一表2");
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
| 1 | PRIMARY | o | NULL | ALL | PRIMARY | NULL | NULL | NULL | 2 | 50 | Using where |
| 1 | PRIMARY | one | NULL | eq_ref | PRIMARY | PRIMARY | 4 | xin-slave.o.one_id | 1 | 100 | Using index |
| 2 | SUBQUERY | t | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
| 3 | SUBQUERY | r | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
二、select_type
select_type
:表示 select
查詢的型別,主要是用於區分各種複雜的查詢,例如:普通查詢
、聯合查詢
、子查詢
等。
1、SIMPLE
SIMPLE
:表示最簡單的 select 查詢語句,也就是在查詢中不包含子查詢或者 union
交併差集等操作。
2、PRIMARY
PRIMARY
:當查詢語句中包含任何複雜的子部分,最外層查詢則被標記為PRIMARY
。
3、SUBQUERY
SUBQUERY
:當 select
或 where
列表中包含了子查詢,該子查詢被標記為:SUBQUERY
。
4、DERIVED
DERIVED
:表示包含在from
子句中的子查詢的select,在我們的 from
列表中包含的子查詢會被標記為derived
。
5、UNION
UNION
:如果union
後邊又出現的select
語句,則會被標記為union
;若 union
包含在 from
子句的子查詢中,外層 select
將被標記為 derived
。
6、UNION RESULT
UNION RESULT
:代表從union
的臨時表中讀取資料,而table
列的<union1,4>
表示用第一個和第四個select
的結果進行union
操作。
mysql> EXPLAIN select t.two_name, ( select one.one_id from one) o from (select two_id,two_name from two where two_name ='') t union (select r.three_name,r.three_id from three r);
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | two | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
| 2 | SUBQUERY | one | NULL | index | NULL | PRIMARY | 4 | NULL | 2 | 100 | Using index |
| 4 | UNION | r | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL |
| NULL | UNION RESULT | <union1,4> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
三、table
查詢的表名,並不一定是真實存在的表,有別名顯示別名,也可能為臨時表,例如上邊的DERIVED
、 <union1,4>
等。
四、partitions
查詢時匹配到的分割槽資訊,對於非分割槽表值為NULL
,當查詢的是分割槽表時,partitions
顯示分割槽表命中的分割槽情況。
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | one | p201801,p201802,p201803,p300012 | index | NULL | PRIMARY | 9 | NULL | 3 | 100 | Using index |
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
五、type
type
:查詢使用了何種型別,它在 SQL
最佳化中是一個非常重要的指標,以下效能從好到壞依次是:system
> const
> eq_ref
> ref
> ref_or_null
> index_merge
> unique_subquery
> index_subquery
> range
> index
> ALL
1、system
system
: 當表僅有一行記錄時(系統表),資料量很少,往往不需要進行磁碟IO,速度非常快。
2、const
const
:表示查詢時命中 primary key
主鍵或者 unique
唯一索引,或者被連線的部分是一個常量(const
)值。這類掃描效率極高,返回資料量少,速度非常快。
mysql> EXPLAIN SELECT * from three where three_id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | three | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
3、eq_ref
eq_ref
:查詢時命中主鍵primary key
或者 unique key
索引, type
就是 eq_ref
。
mysql> EXPLAIN select o.one_name from one o ,two t where o.one_id = t.two_id ;
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
| 1 | SIMPLE | o | NULL | index | PRIMARY | idx_name | 768 | NULL | 2 | 100 | Using index |
| 1 | SIMPLE | t | NULL | eq_ref | PRIMARY | PRIMARY | 4 | xin-slave.o.one_id | 1 | 100 | Using index |
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
4、ref
ref
:區別於eq_ref
,ref
表示使用非唯一性索引,會找到很多個符合條件的行。
mysql> select o.one_id from one o where o.one_name = "xin" ;
+--------+
| one_id |
+--------+
| 1 |
| 3 |
+--------+
mysql> EXPLAIN select o.one_id from one o where o.one_name = "xin" ;
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | o | NULL | ref | idx_name | idx_name | 768 | const | 1 | 100 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
5、ref_or_null
ref_or_null
:這種連線型別類似於 ref,區別在於 MySQL
會額外搜尋包含NULL
值的行。
mysql> EXPLAIN select o.one_id from one o where o.one_name = "xin" OR o.one_name IS NULL;
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
| 1 | SIMPLE | o | NULL | ref_or_null | idx_name | idx_name | 768 | const | 3 | 100 | Using where; Using index |
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
6、index_merge
index_merge
:使用了索引合併最佳化方法,查詢使用了兩個以上的索引。
下邊示例中同時使用到主鍵one_id
和 欄位one_name
的idx_name
索引 。
mysql> EXPLAIN select * from one o where o.one_id >1 and o.one_name ='xin';
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
| 1 | SIMPLE | o | NULL | index_merge | PRIMARY,idx_name | idx_name,PRIMARY | 772,4 | NULL | 1 | 100 | Using intersect(idx_name,PRIMARY); Using where |
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
7、unique_subquery
unique_subquery
:替換下面的 IN
子查詢,子查詢返回不重複的集合。
value IN (SELECT primary_key FROM single_table WHERE some_expr)
8、index_subquery
index_subquery
:區別於unique_subquery
,用於非唯一索引,可以返回重複值。
value IN (SELECT key_column FROM single_table WHERE some_expr)
9、range
range
:使用索引選擇行,僅檢索給定範圍內的行。簡單點說就是針對一個有索引的欄位,給定範圍檢索資料。在where
語句中使用 bettween...and
、<
、>
、<=
、in
等條件查詢 type
都是 range
。
舉個例子:three
表中three_id
為唯一主鍵,user_id
普通欄位未建索引。
mysql> EXPLAIN SELECT * from three where three_id BETWEEN 2 AND 3;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | three | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 1 | 100 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
從結果中看到只有對設定了索引的欄位,做範圍檢索 type
才是 range
。
mysql> EXPLAIN SELECT * from three where user_id BETWEEN 2 AND 3;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | three | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
10、index
index
:Index
與ALL
其實都是讀全表,區別在於index
是遍歷索引樹讀取,而ALL
是從硬碟中讀取。
下邊示例:three_id
為主鍵,不帶 where
條件全表查詢 ,type
結果為index
。
mysql> EXPLAIN SELECT three_id from three ;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | three | NULL | index | NULL | PRIMARY | 4 | NULL | 1 | 100 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
11、ALL
ALL
:將遍歷全表以找到匹配的行,效能最差。
mysql> EXPLAIN SELECT * from two ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | two | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
六、possible_keys
possible_keys
:表示在MySQL
中透過哪些索引,能讓我們在表中找到想要的記錄,一旦查詢涉及到的某個欄位上存在索引,則索引將被列出,但這個索引並不定一會是最終查詢資料時所被用到的索引。具體請參考上邊的例子。
七、key
key
:區別於possible_keys
,key是查詢中實際使用到的索引,若沒有使用索引,顯示為NULL
。具體請參考上邊的例子。
當
type
為index_merge
時,可能會顯示多個索引。
八、key_len
key_len
:表示查詢用到的索引長度(位元組數),原則上長度越短越好 。
- 單列索引,那麼需要將整個索引長度算進去;
- 多列索引,不是所有列都能用到,需要計算查詢中實際用到的列。
注意:
key_len
只計算where
條件中用到的索引長度,而排序和分組即便是用到了索引,也不會計算到key_len
中。
九、ref
ref
:常見的有:const
,func
,null
,欄位名。
- 當使用常量等值查詢,顯示
const
, - 當關聯查詢時,會顯示相應關聯表的
關聯欄位
- 如果查詢條件使用了
表示式
、函式
,或者條件列發生內部隱式轉換,可能顯示為func
- 其他情況
null
十、rows
rows
:以表的統計資訊和索引使用情況,估算要找到我們所需的記錄,需要讀取的行數。
這是評估SQL
效能的一個比較重要的資料,mysql
需要掃描的行數,很直觀的顯示 SQL
效能的好壞,一般情況下 rows
值越小越好。
mysql> EXPLAIN SELECT * from three;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | three | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
十一、filtered
filtered
這個是一個百分比的值,表裡符合條件的記錄數的百分比。簡單點說,這個欄位表示儲存引擎返回的資料在經過過濾後,剩下滿足條件的記錄數量的比例。
在MySQL.5.7
版本以前想要顯示filtered
需要使用explain extended
命令。MySQL.5.7
後,預設explain
直接顯示partitions
和filtered
的資訊。
十二、Extra
Extra
:不適合在其他列中顯示的資訊,Explain
中的很多額外的資訊會在 Extra
欄位顯示。
1、Using index
Using index
:我們在相應的 select
操作中使用了覆蓋索引,通俗一點講就是查詢的列被索引覆蓋,使用到覆蓋索引查詢速度會非常快,SQl
最佳化中理想的狀態。
什麼又是覆蓋索引?
一條 SQL
只需要透過索引就可以返回,我們所需要查詢的資料(一個或幾個欄位),而不必透過二級索引,查到主鍵之後再透過主鍵查詢整行資料(select *
)。
one_id
表為主鍵
mysql> EXPLAIN SELECT one_id from one ;
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
| 1 | SIMPLE | one | NULL | index | NULL | idx_two_id | 5 | NULL | 3 | 100 | Using index |
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
注意:想要使用到覆蓋索引,我們在 select
時只取出需要的欄位,不可select *
,而且該欄位建了索引。
mysql> EXPLAIN SELECT * from one ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | one | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
2、Using where
Using where
:查詢時未找到可用的索引,進而透過where
條件過濾獲取所需資料,但要注意的是並不是所有帶where
語句的查詢都會顯示Using where
。
下邊示例create_time
並未用到索引,type
為 ALL
,即MySQL
透過全表掃描後再按where
條件篩選資料。
mysql> EXPLAIN SELECT one_name from one where create_time ='2020-05-18';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | one | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
3、Using temporary
Using temporary
:表示查詢後結果需要使用臨時表來儲存,一般在排序或者分組查詢時用到。
mysql> EXPLAIN SELECT one_name from one where one_id in (1,2) group by one_name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | one | NULL | range| NULL | NULL | NULL | NULL | 3 | 33.33 | Using where; Using temporary; Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
4、Using filesort
Using filesort
:表示無法利用索引完成的排序操作,也就是ORDER BY
的欄位沒有索引,通常這樣的SQL都是需要最佳化的。
mysql> EXPLAIN SELECT one_id from one ORDER BY create_time;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | SIMPLE | one | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
如果ORDER BY
欄位有索引就會用到覆蓋索引,相比執行速度快很多。
mysql> EXPLAIN SELECT one_id from one ORDER BY one_id;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | one | NULL | index | NULL | PRIMARY | 4 | NULL | 3 | 100 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
5、Using join buffer
Using join buffer
:在我們聯表查詢的時候,如果表的連線條件沒有用到索引,需要有一個連線緩衝區來儲存中間結果。
先看一下有索引的情況:連線條件 one_name
、two_name
都用到索引。
mysql> EXPLAIN SELECT one_name from one o,two t where o.one_name = t.two_name;
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
| 1 | SIMPLE | o | NULL | index | idx_name | idx_name | 768 | NULL | 3 | 100 | Using where; Using index |
| 1 | SIMPLE | t | NULL | ref | idx_name | idx_name | 768 | xin-slave.o.one_name | 1 | 100 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
接下來刪掉 連線條件 one_name
、two_name
的欄位索引。發現Extra
列變成 Using join buffer
,type
均為全表掃描,這也是SQL
最佳化中需要注意的地方。
mysql> EXPLAIN SELECT one_name from one o,two t where o.one_name = t.two_name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | t | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL |
| 1 | SIMPLE | o | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
6、Impossible where
Impossible where
:表示在我們用不太正確的where
語句,導致沒有符合條件的行。
mysql> EXPLAIN SELECT one_name from one WHERE 1=2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
7、No tables used
No tables used
:我們的查詢語句中沒有FROM
子句,或者有 FROM DUAL
子句。
mysql> EXPLAIN select now();
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
Extra
列的資訊非常非常多,這裡就不再一一列舉了,詳見 MySQL
官方文件 :dev.mysql.com/doc/refman/5.7/en/ex...
總結
上邊只是簡單介紹了下 Explain
執行計劃各個列的含義,瞭解它不僅僅是要應付面試,在實際開發中也經常會用到。比如對慢SQL
進行分析,如果連執行計劃結果都不會看,那還談什麼SQL
最佳化呢?
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