重點是第二種用法，需要深入的瞭解。

先看一個例子：

mysql> explain select * from t_order;
+----+-------------+---------+------+---------------+------+---------+------+--------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+---------------+------+---------+------+--------+-------+
| 1 | SIMPLE | t_order | ALL | NULL | NULL | NULL | NULL | 100453 | |
+----+-------------+---------+------+---------------+------+---------+------+--------+-------+
1 row in set (0.03 sec)

加上extended後之後：

mysql> explain extended select * from t_order;
+----+-------------+---------+------+---------------+------+---------+------+--------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------+---------------+------+---------+------+--------+----------+-------+
| 1 | SIMPLE | t_order | ALL | NULL | NULL | NULL | NULL | 100453 | 100.00 | |
+----+-------------+---------+------+---------------+------+---------+------+--------+----------+-------+
1 row in set, 1 warning (0.00 sec)

有必要解釋一下這個長長的表格裡每一列的含義：

id	SELECT識別符。這是SELECT的查詢序列號
select_type	SELECT型別,可以為以下任何一種: SIMPLE:簡單SELECT(不使用UNION或子查詢) PRIMARY:最外面的SELECT UNION:UNION中的第二個或後面的SELECT語句 DEPENDENT UNION:UNION中的第二個或後面的SELECT語句,取決於外面的查詢 UNION RESULT:UNION 的結果 SUBQUERY:子查詢中的第一個SELECT DEPENDENT SUBQUERY:子查詢中的第一個SELECT,取決於外面的查詢 DERIVED:匯出表的SELECT(FROM子句的子查詢)
table	輸出的行所引用的表
type	聯接型別。下面給出各種聯接型別,按照從最佳型別到最壞型別進行排序: system:表僅有一行(=系統表)。這是const聯接型別的一個特例。 const:表最多有一個匹配行,它將在查詢開始時被讀取。因為僅有一行,在這行的列值可被最佳化器剩餘部分認為是常數。const表很快,因為它們只讀取一次! eq_ref:對於每個來自於前面的表的行組合,從該表中讀取一行。這可能是最好的聯接型別,除了const型別。 ref:對於每個來自於前面的表的行組合,所有有匹配索引值的行將從這張表中讀取。 ref_or_null:該聯接型別如同ref,但是新增了MySQL可以專門搜尋包含NULL值的行。 index_merge:該聯接型別表示使用了索引合併最佳化方法。 unique_subquery:該型別替換了下面形式的IN子查詢的ref: value IN (SELECT primary_key FROM single_table WHERE some_expr) unique_subquery是一個索引查詢函式,可以完全替換子查詢,效率更高。 index_subquery:該聯接型別類似於unique_subquery。可以替換IN子查詢,但只適合下列形式的子查詢中的非唯一索引: value IN (SELECT key_column FROM single_table WHERE some_expr) range:只檢索給定範圍的行,使用一個索引來選擇行。 index:該聯接型別與ALL相同,除了只有索引樹被掃描。這通常比ALL快,因為索引檔案通常比資料檔案小。 ALL:對於每個來自於先前的表的行組合,進行完整的表掃描。
possible_keys	指出MySQL能使用哪個索引在該表中找到行
key	顯示MySQL實際決定使用的鍵(索引)。如果沒有選擇索引,鍵是NULL。
key_len	顯示MySQL決定使用的鍵長度。如果鍵是NULL,則長度為NULL。
ref	顯示使用哪個列或常數與key一起從表中選擇行。
rows	顯示MySQL認為它執行查詢時必須檢查的行數。多行之間的資料相乘可以估算要處理的行數。
filtered	顯示了透過條件過濾出的行數的百分比估計值。
Extra	該列包含MySQL解決查詢的詳細資訊 Distinct:MySQL發現第1個匹配行後,停止為當前的行組合搜尋更多的行。 Not exists:MySQL能夠對查詢進行LEFT JOIN最佳化,發現1個匹配LEFT JOIN標準的行後,不再為前面的的行組合在該表內檢查更多的行。 range checked for each record (index map: #):MySQL沒有發現好的可以使用的索引,但發現如果來自前面的表的列值已知,可能部分索引可以使用。 Using filesort:MySQL需要額外的一次傳遞,以找出如何按排序順序檢索行。 Using index:從只使用索引樹中的資訊而不需要進一步搜尋讀取實際的行來檢索表中的列資訊。 Using temporary:為了解決查詢,MySQL需要建立一個臨時表來容納結果。 Using where:WHERE 子句用於限制哪一個行匹配下一個表或傳送到客戶。 Using sort_union(...), Using union(...), Using intersect(...):這些函式說明如何為index_merge聯接型別合併索引掃描。 Using index for group-by:類似於訪問表的Using index方式,Using index for group-by表示MySQL發現了一個索引,可以用來查詢GROUP BY或DISTINCT查詢的所有列,而不要額外搜尋硬碟訪問實際的表。

一.select_type的說明

1.UNION:

當透過union來連線多個查詢結果時，第二個之後的select其select_type為UNION。

mysql> explain select * from t_order where order_id=100 union select * from t_order where order_id=200;
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-------+
| 1 | PRIMARY | t_order | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| 2 | UNION | t_order | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-------+
3 rows in set (0.34 sec)

2.DEPENDENT UNION與DEPENDENT SUBQUERY:

當union作為子查詢時，其中第二個union的select_type就是DEPENDENT UNION。
第一個子查詢的select_type則是DEPENDENT SUBQUERY。

mysql> explain select * from t_order where order_id in (select order_id from t_order where order_id=100 union select order_id from t_order where order_id=200);
+----+--------------------+------------+-------+---------------+---------+---------+-------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------+-------+---------------+---------+---------+-------+--------+-------------+
| 1 | PRIMARY | t_order | ALL | NULL | NULL | NULL | NULL | 100453 | Using where |
| 2 | DEPENDENT SUBQUERY | t_order | const | PRIMARY | PRIMARY | 4 | const | 1 | Using index |
| 3 | DEPENDENT UNION | t_order | const | PRIMARY | PRIMARY | 4 | const | 1 | Using index |
| NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------------+------------+-------+---------------+---------+---------+-------+--------+-------------+
4 rows in set (0.03 sec)

3.SUBQUERY:

子查詢中的第一個select其select_type為SUBQUERY。

mysql> explain select * from t_order where order_id=(select order_id from t_order where order_id=100);
+----+-------------+---------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | PRIMARY | t_order | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| 2 | SUBQUERY | t_order | const | PRIMARY | PRIMARY | 4 | | 1 | Using index |
+----+-------------+---------+-------+---------------+---------+---------+-------+------+-------------+
2 rows in set (0.03 sec)

4.DERIVED:

當子查詢是from子句時，其select_type為DERIVED。

mysql> explain select * from (select order_id from t_order where order_id=100) a;
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | t_order | const | PRIMARY | PRIMARY | 4 | | 1 | Using index |
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+
2 rows in set (0.03 sec)

二.type的說明

1.system，const

見上面4.DERIVED的例子。其中第一行的type就是為system，第二行是const，這兩種聯接型別是最快的。

2.eq_ref

在t_order表中的order_id是主鍵，t_order_ext表中的order_id也是主鍵，該表可以認為是訂單表的補充資訊表，他們的關係是1對1，在下面的例子中可以看到b表的連線型別是eq_ref，這是極快的聯接型別。

mysql> explain select * from t_order a,t_order_ext b where a.order_id=b.order_id;
+----+-------------+-------+--------+---------------+---------+---------+-----------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+-----------------+------+-------------+
| 1 | SIMPLE | b | ALL | order_id | NULL | NULL | NULL | 1 | |
| 1 | SIMPLE | a | eq_ref | PRIMARY | PRIMARY | 4 | test.b.order_id | 1 | Using where |
+----+-------------+-------+--------+---------------+---------+---------+-----------------+------+-------------+
2 rows in set (0.00 sec)

3.ref

下面的例子在上面的例子上略作了修改，加上了條件。此時b表的聯接型別變成了ref。因為所有與a表中order_id=100的匹配記錄都將會從b表獲取。這是比較常見的聯接型別。

mysql> explain select * from t_order a,t_order_ext b where a.order_id=b.order_id and a.order_id=100;
+----+-------------+-------+-------+---------------+----------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+-------+------+-------+
| 1 | SIMPLE | a | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| 1 | SIMPLE | b | ref | order_id | order_id | 4 | const | 1 | |
+----+-------------+-------+-------+---------------+----------+---------+-------+------+-------+
2 rows in set (0.00 sec)

4.ref_or_null

user_id欄位是一個可以為空的欄位，並對該欄位建立了一個索引。在下面的查詢中可以看到聯接型別為ref_or_null，這是mysql為含有null的欄位專門做的處理。在我們的表設計中應當儘量避免索引欄位為NULL，因為這會額外的耗費mysql的處理時間來做最佳化。

mysql> explain select * from t_order where user_id=100 or user_id is null;
+----+-------------+---------+-------------+---------------+---------+---------+-------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------------+---------------+---------+---------+-------+-------+-------------+
| 1 | SIMPLE | t_order | ref_or_null | user_id | user_id | 5 | const | 50325 | Using where |
+----+-------------+---------+-------------+---------------+---------+---------+-------+-------+-------------+
1 row in set (0.00 sec)

5.index_merge

經常出現在使用一張表中的多個索引時。mysql會將多個索引合併在一起，如下例:

mysql> explain select * from t_order where order_id=100 or user_id=10;
+----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+-------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+-------------------------------------------+
| 1 | SIMPLE | t_order | index_merge | PRIMARY,user_id | PRIMARY,user_id | 4,5 | NULL | 2 | Using union(PRIMARY,user_id); Using where |
+----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+-------------------------------------------+
1 row in set (0.09 sec)

6.unique_subquery

該聯接型別用於替換value IN (SELECT primary_key FROM single_table WHERE some_expr)這樣的子查詢的ref。注意ref列，其中第二行顯示的是func，表明unique_subquery是一個函式，而不是一個普通的ref。

mysql> explain select * from t_order where order_id in (select order_id from t_order where user_id=10);
+----+--------------------+---------+-----------------+-----------------+---------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+---------+-----------------+-----------------+---------+---------+------+--------+-------------+
| 1 | PRIMARY | t_order | ALL | NULL | NULL | NULL | NULL | 100649 | Using where |
| 2 | DEPENDENT SUBQUERY | t_order | unique_subquery | PRIMARY,user_id | PRIMARY | 4 | func | 1 | Using where |
+----+--------------------+---------+-----------------+-----------------+---------+---------+------+--------+-------------+
2 rows in set (0.00 sec)

7.index_subquery

該聯接型別與上面的太像了，唯一的差別就是子查詢查的不是主鍵而是非唯一索引。

mysql> explain select * from t_order where user_id in (select user_id from t_order where order_id>10);
+----+--------------------+---------+----------------+-----------------+---------+---------+------+--------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+---------+----------------+-----------------+---------+---------+------+--------+--------------------------+
| 1 | PRIMARY | t_order | ALL | NULL | NULL | NULL | NULL | 100649 | Using where |
| 2 | DEPENDENT SUBQUERY | t_order | index_subquery | PRIMARY,user_id | user_id | 5 | func | 50324 | Using index; Using where |
+----+--------------------+---------+----------------+-----------------+---------+---------+------+--------+--------------------------+
2 rows in set (0.00 sec)

8.range

按指定的範圍進行檢索，很常見。

mysql> explain select * from t_order where user_id in (100,200,300);
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | t_order | range | user_id | user_id | 5 | NULL | 3 | Using where |
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

9.index

在進行統計時非常常見，此聯接型別實際上會掃描索引樹，僅比ALL快些。

mysql> explain select count(*) from t_order;
+----+-------------+---------+-------+---------------+---------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------+---------------+---------+---------+------+--------+-------------+
| 1 | SIMPLE | t_order | index | NULL | user_id | 5 | NULL | 100649 | Using index |
+----+-------------+---------+-------+---------------+---------+---------+------+--------+-------------+
1 row in set (0.00 sec)

10.ALL

完整的掃描全表，最慢的聯接型別，儘可能的避免。

mysql> explain select * from t_order;
+----+-------------+---------+------+---------------+------+---------+------+--------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+---------------+------+---------+------+--------+-------+
| 1 | SIMPLE | t_order | ALL | NULL | NULL | NULL | NULL | 100649 | |
+----+-------------+---------+------+---------------+------+---------+------+--------+-------+
1 row in set (0.00 sec)

三.extra的說明

1.Distinct

MySQL發現第1個匹配行後,停止為當前的行組合搜尋更多的行。對於此項沒有找到合適的例子，求指點。

2.Not exists

因為b表中的order_id是主鍵，不可能為NULL，所以mysql在用a表的order_id掃描t_order表，並查詢b表的行時，如果在b表發現一個匹配的行就不再繼續掃描b了，因為b表中的order_id欄位不可能為NULL。這樣避免了對b表的多次掃描。

mysql> explain select count(1) from t_order a left join t_order_ext b on a.order_id=b.order_id where b.order_id is null;
+----+-------------+-------+-------+---------------+--------------+---------+-----------------+--------+--------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+--------------+---------+-----------------+--------+--------------------------------------+
| 1 | SIMPLE | a | index | NULL | express_type | 1 | NULL | 100395 | Using index |
| 1 | SIMPLE | b | ref | order_id | order_id | 4 | test.a.order_id | 1 | Using where; Using index; Not exists |
+----+-------------+-------+-------+---------------+--------------+---------+-----------------+--------+--------------------------------------+
2 rows in set (0.01 sec)

3.Range checked for each record

這種情況是mysql沒有發現好的索引可用，速度比沒有索引要快得多。

mysql> explain select * from t_order t, t_order_ext s where s.order_id>=t.order_id and s.order_id<=t.order_id and t.express_type>5;
+----+-------------+-------+-------+----------------------+--------------+---------+------+------+------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+----------------------+--------------+---------+------+------+------------------------------------------------+
| 1 | SIMPLE | t | range | PRIMARY,express_type | express_type | 1 | NULL | 1 | Using where |
| 1 | SIMPLE | s | ALL | order_id | NULL | NULL | NULL | 1 | Range checked for each record (index map: 0x1) |
+----+-------------+-------+-------+----------------------+--------------+---------+------+------+------------------------------------------------+
2 rows in set (0.00 sec)

4.Using filesort

在有排序子句的情況下很常見的一種情況。此時mysql會根據聯接型別瀏覽所有符合條件的記錄，並儲存排序關鍵字和行指標，然後排序關鍵字並按順序檢索行。

mysql> explain select * from t_order order by express_type;
+----+-------------+---------+------+---------------+------+---------+------+--------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+---------------+------+---------+------+--------+----------------+
| 1 | SIMPLE | t_order | ALL | NULL | NULL | NULL | NULL | 100395 | Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+--------+----------------+
1 row in set (0.00 sec)

5.Using index

這是效能很高的一種情況。當查詢所需的資料可以直接從索引樹中檢索到時，就會出現。上面的例子中有很多這樣的例子，不再多舉例了。

6.Using temporary

發生這種情況一般都是需要進行最佳化的。mysql需要建立一張臨時表用來處理此類查詢。

mysql> explain select * from t_order a left join t_order_ext b on a.order_id=b.order_id group by b.order_id;
+----+-------------+-------+------+---------------+----------+---------+-----------------+--------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+----------+---------+-----------------+--------+---------------------------------+
| 1 | SIMPLE | a | ALL | NULL | NULL | NULL | NULL | 100395 | Using temporary; Using filesort |
| 1 | SIMPLE | b | ref | order_id | order_id | 4 | test.a.order_id | 1 | |
+----+-------------+-------+------+---------------+----------+---------+-----------------+--------+---------------------------------+
2 rows in set (0.00 sec)

7.Using where

當有where子句時，extra都會有說明。

8.Using sort_union(...)/Using union(...)/Using intersect(...)

下面的例子中user_id是一個檢索範圍，此時mysql會使用sort_union函式來進行索引的合併。而當user_id是一個固定值時，請參看上面type說明5.index_merge的例子，此時會使用union函式進行索引合併。

mysql> explain select * from t_order where order_id=100 or user_id>10;
+----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+------------------------------------------------+
| 1 | SIMPLE | t_order | index_merge | PRIMARY,user_id | user_id,PRIMARY | 5,4 | NULL | 2 | Using sort_union(user_id,PRIMARY); Using where |
+----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+------------------------------------------------+
1 row in set (0.00 sec)

對於Using intersect的例子可以參看下例，user_id與express_type發生了索引交叉合併。

mysql> explain select * from t_order where express_type=1 and user_id=100;
+----+-------------+---------+-------------+----------------------+----------------------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------------+----------------------+----------------------+---------+------+------+----------------------------------------------------+
| 1 | SIMPLE | t_order | index_merge | user_id,express_type | user_id,express_type | 5,1 | NULL | 1 | Using intersect(user_id,express_type); Using where |
+----+-------------+---------+-------------+----------------------+----------------------+---------+------+------+----------------------------------------------------+
1 row in set (0.00 sec)

9.Using index for group-by

表明可以在索引中找到分組所需的所有資料，不需要查詢實際的表。

mysql> explain select user_id from t_order group by user_id;
+----+-------------+---------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------+---------------+---------+---------+------+------+--------------------------+
| 1 | SIMPLE | t_order | range | NULL | user_id | 5 | NULL | 3 | Using index for group-by |
+----+-------------+---------+-------+---------------+---------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

除了上面的三個說明，還需要注意rows的數值，多行之間的數值是乘積的關係，可以估算大概要處理的行數，如果乘積很大，那就很有最佳化的必要了。

mysql explain用法和結果的含義

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