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MySQL的子查詢及相關(guān)優(yōu)化學習教程

 更新時間：2015年11月17日 10:09:05   作者：gongwen  
這篇文章主要介紹了MySQL的子查詢及相關(guān)優(yōu)化學習教程,使用子查詢時需要注意其對數(shù)據(jù)庫性能的影響,需要的朋友可以參考下

一、子查詢
1、where型子查詢
（把內(nèi)層查詢結(jié)果當作外層查詢的比較條件）

#不用order by 來查詢最新的商品
select goods_id,goods_name from goods where goods_id = (select max(goods_id) from goods);

#取出每個欄目下最新的產(chǎn)品(goods_id唯一)
select cat_id,goods_id,goods_name from goods where goods_id in(select max(goods_id) from goods group by cat_id);

2、from型子查詢
(把內(nèi)層的查詢結(jié)果供外層再次查詢)
#用子查詢查出掛科兩門及以上的同學的平均成績
思路：

#先查出哪些同學掛科兩門以上
select name,count(*) as gk from stu where score < 60 having gk >=2;
#以上查詢結(jié)果，我們只要名字就可以了，所以再取一次名字
select name from (select name,count(*) as gk from stu having gk >=2) as t;
#找出這些同學了，那么再計算他們的平均分
select name,avg(score) from stu where name in (select name from (select name,count(*) as gk from stu having gk >=2) as t) group by name;

3、exists型子查詢
（把外層查詢結(jié)果拿到內(nèi)層，看內(nèi)層的查詢是否成立）

#查詢哪些欄目下有商品，欄目表category,商品表goods
select cat_id,cat_name from category where exists(select * from goods where goods.cat_id = category.cat_id);

二、優(yōu)化
從句式的形式看，子查詢分為特殊格式子查詢和非特殊格式子查詢，特殊格式的子查詢中又包括IN、ALL、ANY、SOME、EXISTS等類型的子查詢，對于有的類型的子查詢，MySQL有的支持優(yōu)化，有的不支持，具體情況如下。

 

示例一，MySQL不支持對EXISTS類型的子查詢的優(yōu)化：

EXISTS類型的相關(guān)子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE EXISTS (SELECT 1 FROM t2 WHERE t1.a1= t2.a2 AND t2.a2>10);


+----+--------------------+-------+------+------+-------------+
| id | select_type    | table | type | key | Extra    |

+----+--------------------+-------+------+------+-------------+

| 1 | PRIMARY      | t1  | ALL | NULL | Using where |

| 2 | DEPENDENT SUBQUERY | t2  | ALL | NULL | Using where |

+----+--------------------+-------+------+------+-------------+

2 rows in set, 2 warnings (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where exists(/* select#2 */

  select 1

  from `test`.`t2`

  where ((`test`.`t1`.`a1` = `test`.`t2`.`a2`) and (`test`.`t2`.`a2` > 10))

)

從查詢執(zhí)行計劃看，子查詢存在，MySQL沒有進一步做子查詢的優(yōu)化工作。

另外的一個EXISTS類型的相關(guān)子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE EXISTS (SELECT 1 FROM t2 WHERE t1.b1= t2.b2 AND t1.a1=10);


+----+--------------------+-------+------+------+-------------+

| id | select_type    | table | type | key | Extra    |

+----+--------------------+-------+------+------+-------------+

| 1 | PRIMARY      | t1  | ALL | NULL | Using where |

| 2 | DEPENDENT SUBQUERY | t2  | ALL | NULL | Using where |

+----+--------------------+-------+------+------+-------------+

2 rows in set, 3 warnings (0.02 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where exists(/* select#2 */

  select 1

  from `test`.`t2`

  where ((`test`.`t1`.`b1` = `test`.`t2`.`b2`) and (`test`.`t1`.`a1` = 10))

)

從查詢執(zhí)行計劃看，子查詢存在，MySQL沒有進一步做子查詢的優(yōu)化工作。

 

示例二，MySQL不支持對NOT EXISTS類型的子查詢的優(yōu)化：

NOT EXISTS類型的相關(guān)子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE t1.a1= t2.a2 AND t2.a2>10);


+----+--------------------+-------+------+------+-------------+

| id | select_type    | table | type | key | Extra    |

+----+--------------------+-------+------+------+-------------+

| 1 | PRIMARY      | t1  | ALL | NULL | Using where |

| 2 | DEPENDENT SUBQUERY | t2  | ALL | NULL | Using where |

+----+--------------------+-------+------+------+-------------+

2 rows in set, 2 warnings (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (not(exists(

  /* select#2 */ select 1

  from `test`.`t2`

  where ((`test`.`t1`.`a1` = `test`.`t2`.`a2`) and (`test`.`t2`.`a2` > 10))))

)

從查詢執(zhí)行計劃看，子查詢存在，MySQL沒有進一步做子查詢的優(yōu)化工作。

 

另外的一個NOT EXISTS類型的相關(guān)子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE t1.b1= t2.b2 AND t1.a1=10);


+----+--------------------+-------+------+------+-------------+

| id | select_type    | table | type | key | Extra    |

+----+--------------------+-------+------+------+-------------+

| 1 | PRIMARY      | t1  | ALL | NULL | Using where |

| 2 | DEPENDENT SUBQUERY | t2  | ALL | NULL | Using where |

+----+--------------------+-------+------+------+-------------+

2 rows in set, 3 warnings (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (not(exists(

  /* select#2 */ select 1

  from `test`.`t2`

  where ((`test`.`t1`.`b1` = `test`.`t2`.`b2`) and (`test`.`t1`.`a1` = 10))))

)

從查詢執(zhí)行計劃看，子查詢存在，MySQL沒有進一步做子查詢的優(yōu)化工作。

 

示例三，MySQL支持對IN類型的子查詢的優(yōu)化，按也有不支持的情況存在：

IN非相關(guān)子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 IN (SELECT a2 FROM t2 WHERE t2.a2>10);


+----+--------------+-------------+------+------+----------------------------------------------------+

| id | select_type | table    | type | key | Extra  |

+----+--------------+-------------+------+------+----------------------------------------------------+

| 1 | SIMPLE    | <subquery2> | ALL | NULL | NULL  |

| 1 | SIMPLE    | t1     | ALL | NULL | Using where; Using join buffer (Block Nested Loop) |

| 2 | MATERIALIZED | t2     | ALL | NULL | Using where  |

+----+--------------+-------------+------+------+----------------------------------------------------+

3 rows in set, 1 warning (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1` semi join (`test`.`t2`)

where ((`test`.`t1`.`a1` = `<subquery2>`.`a2`) and (`test`.`t2`.`a2` > 10))

從查詢執(zhí)行計劃看，表t2被物化后，與表t1執(zhí)行了半連接（semi join）。盡管有“subquery2”這樣的內(nèi)容看起來是子查詢，但是表t2已經(jīng)被上拉到表t1層執(zhí)行了半連接，所以MySQL支持IN子查詢優(yōu)化為半連接操作。

 

另外一個IN非相關(guān)子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 IN (SELECT a2 FROM t2 WHERE t2.a2=10);


+----+--------------+-------------+------+------+----------------------------------------------------+

| id | select_type | table    | type | key | Extra  |

+----+--------------+-------------+------+------+----------------------------------------------------+

| 1 | SIMPLE    | <subquery2> | ALL | NULL | Using where  |

| 1 | SIMPLE    | t1     | ALL | NULL | Using where; Using join buffer (Block Nested Loop) |

| 2 | MATERIALIZED | t2     | ALL | NULL | Using where  |

+----+--------------+-------------+------+------+----------------------------------------------------+

3 rows in set, 1 warning (0.02 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1` semi join (`test`.`t2`)

where ((`<subquery2>`.`a2` = 10) and (`test`.`t1`.`a1` = 10) and (`test`.`t2`.`a2` = 10))

從查詢執(zhí)行計劃看，子查詢不存在，表t1和t2直接做了塊嵌套循環(huán)半連接（Block Nested Loop），把子查詢上拉到父查詢中用嵌套循環(huán)半連接完成IN操作。另外，由于子查詢上拉，使得增加連接條件“a1=a2”，而原先的條件“a2=10”可以利用常量傳遞優(yōu)化技術(shù)，使得“a1=a2=10”，所以查詢執(zhí)行計劃中，兩個索引掃描的條件分別為：a1 = 10、a2 = 10。

 

另外一個IN非相關(guān)子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 IN (SELECT a2 FROM t2 WHERE t1.a1=10);


+----+-------------+-------+------+------------------------------------------------------------------+

| id | select_type | table | type | Extra      |

+----+-------------+-------+------+------------------------------------------------------------------+

| 1 | SIMPLE   | t2  | ALL | Using where; Start temporary      |

| 1 | SIMPLE   | t1  | ALL | Using where; End temporary; Using join buffer (Block Nested Loop)|

+----+-------------+-------+------+------------------------------------------------------------------+

2 rows in set, 2 warnings (0.00 sec)

被查詢優(yōu)化器處理后的語句為：
/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1` semi join (`test`.`t2`)

where ((`test`.`t2`.`a2` = 10) and (`test`.`t1`.`a1` = 10))

從查詢執(zhí)行計劃看，子子查詢不存在，表t1和t2直接做了塊嵌套循環(huán)連接（Block Nested Loop），但屬于半連接操作（semi join），把子查詢上拉到父查詢中用嵌套循環(huán)半連接完成IN操作。

 

示例四，MySQL支持對NOT IN類型的子查詢的優(yōu)化

NOT IN非相關(guān)子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 NOT IN (SELECT a2 FROM t2 WHERE t2.a2>10);


+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.02 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

`test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (not(<in_optimizer>(

  `test`.`t1`.`a1`,`test`.`t1`.`a1` in (

    <materialize> (/* select#2 */

      select `test`.`t2`.`a2`

      from `test`.`t2`

      where (`test`.`t2`.`a2` > 10)

      having 1

    ),

    <primary_index_lookup>(

      `test`.`t1`.`a1` in <temporary table> on <auto_key>

      where ((`test`.`t1`.`a1` = `materialized-subquery`.`a2`))

    )

   )

  ))

)

從查詢執(zhí)行計劃看，表t2做了子查詢（SUBQUERY）。而子查詢被物化（materialize）。所以，MySQL對于NOT IN子查詢采用了物化的優(yōu)化方式，但不支持子查詢的消除。

 

另外一個NOT IN非相關(guān)子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 NOT IN (SELECT a2 FROM t2 WHERE t2.a2=10);


+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,`test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (not(<in_optimizer>(

  `test`.`t1`.`a1`,`test`.`t1`.`a1` in (

    <materialize> (/* select#2 */

      select `test`.`t2`.`a2`

      from `test`.`t2`

      where (`test`.`t2`.`a2` = 10)

      having 1

    ),

    <primary_index_lookup>(

      `test`.`t1`.`a1` in <temporary table> on <auto_key>

      where ((`test`.`t1`.`a1` = `materialized-subquery`.`a2`))

    )

  )

  ))

)

從查詢執(zhí)行計劃看，表t2做了子查詢（SUBQUERY）。而子查詢被物化（materialize）。所以，MySQL對于NOT IN子查詢采用了物化的優(yōu)化方式，但不支持子查詢的消除。

 

示例五，MySQL支持對ALL類型的子查詢的優(yōu)化：

不相關(guān)的ALL子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 >ALL (SELECT a2 FROM t2 WHERE t2.a2>10);


+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,`test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where <not>((`test`.`t1`.`a1` <= <max>(

  /* select#2 */

  select `test`.`t2`.`a2`

  from `test`.`t2`

  where (`test`.`t2`.`a2` > 10)

  )

))

從查詢執(zhí)行計劃看，出現(xiàn)了子查詢（SUBQUERY），但是，子查詢被“<= <max>”操作符限制，而子查詢中的被查詢列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“>ALL”式的子查詢優(yōu)化，子查詢只被執(zhí)行一次即可求得最大值。

 

不相關(guān)的ALL子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 =ALL (SELECT a2 FROM t2 WHERE t2.a2=10);


+----+--------------------+-------+------+------+-------------+

| id | select_type    | table | type | key | Extra    |

+----+--------------------+-------+------+------+-------------+

| 1 | PRIMARY      | t1  | ALL | NULL | Using where |

| 2 | DEPENDENT SUBQUERY | t2  | ALL | NULL | Using where |

+----+--------------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,`test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where <not>(<in_optimizer>(

  `test`.`t1`.`a1`,<exists>(

    /* select#2 */ select 1 from `test`.`t2`

    where ((`test`.`t2`.`a2` = 10) and

      <if>(outer_field_is_not_null,

        ((<cache>(`test`.`t1`.`a1`) <> 10) or <cache>(isnull(10))),

        true

      )

    )

    having <if>(outer_field_is_not_null, <is_not_null_test>(`test`.`t2`.`a2`), true)

  )

))

從查詢執(zhí)行計劃看，出現(xiàn)了子查詢（SUBQUERY），但是被查詢優(yōu)化器處理后的語句中包含“exists”，這表明MySQL對于“=ALL”式的子查詢優(yōu)化用“EXISTS strategy”方式優(yōu)化，所以MySQL支持“=ALL”式的子查詢優(yōu)化。

 

不相關(guān)的ALL子查詢，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 <ALL (SELECT a2 FROM t2 WHERE t2.a2=10);


+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,`test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where <not>((`test`.`t1`.`a1` >= <min>

  (/* select#2 */

    select `test`.`t2`.`a2`

    from `test`.`t2`

    where (`test`.`t2`.`a2` = 10)

  )

))

從查詢執(zhí)行計劃看，出現(xiàn)了子查詢（SUBQUERY），但是，子查詢被“>= <min>”操作符限制，而子查詢中的被查詢列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“<ALL”式的子查詢優(yōu)化，子查詢只被執(zhí)行一次即可求得最小值。

 

示例六，MySQL支持對SOME類型的子查詢的優(yōu)化：

使用了“>SOME”式子的子查詢被優(yōu)化，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 >SOME (SELECT a2 FROM t2 WHERE t2.a2>10);


+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.05 sec)

被查詢優(yōu)化器處理后的語句為：

 /* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

   `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where <nop>((`test`.`t1`.`a1` > (

  /* select#2 */

  select min(`test`.`t2`.`a2`)

  from `test`.`t2`

  where (`test`.`t2`.`a2` > 10)

)))

從查詢執(zhí)行計劃看，出現(xiàn)了子查詢（SUBQUERY），但是，子查詢被“min”函數(shù)限制，而子查詢中的被查詢列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“>SOME”式的子查詢優(yōu)化，子查詢只被執(zhí)行一次即可求得最大值。

 

使用了“=SOME”式子的子查詢被優(yōu)化，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 =SOME (SELECT a2 FROM t2 WHERE t2.a2=10);


+----+--------------+-------------+------+------+----------------------------------------------------+

| id | select_type | table    | type | key | Extra  |

+----+--------------+-------------+------+------+----------------------------------------------------+

| 1 | SIMPLE    | <subquery2> | ALL | NULL | Using where  |

| 1 | SIMPLE    | t1     | ALL | NULL | Using where; Using join buffer (Block Nested Loop) |

| 2 | MATERIALIZED | t2     | ALL | NULL | Using where  |

+----+--------------+-------------+------+------+----------------------------------------------------+

3 rows in set, 1 warning (0.01 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

`test`.`t1`.`b1` AS `b1`

from `test`.`t1` semi join (`test`.`t2`)

where ((`<subquery2>`.`a2` = 10) and (`test`.`t1`.`a1` = 10) and (`test`.`t2`.`a2` = 10))

從查詢執(zhí)行計劃看，沒有出現(xiàn)了子查詢，表t2被物化，與表t1進行了半連接。

 

使用了“<SOME”式子的子查詢被優(yōu)化，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 <SOME (SELECT a2 FROM t2 WHERE t2.a2=10);


+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where <nop>(

  (

    `test`.`t1`.`a1` < (/* select#2 */

      select max(`test`.`t2`.`a2`)

      from `test`.`t2`

      where (`test`.`t2`.`a2` = 10)

    )

  )

)

從查詢執(zhí)行計劃看，出現(xiàn)了子查詢（SUBQUERY），但是，子查詢被“max”函數(shù)限制，而子查詢中的被查詢列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“<SOME”式的子查詢優(yōu)化，子查詢只被執(zhí)行一次即可求得最大值。

 

示例七，MySQL支持對ANY類型的子查詢的優(yōu)化：

使用了“>ANY”式子的子查詢被優(yōu)化，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 >ANY (SELECT a2 FROM t2 WHERE t2.a2>10);


+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where <nop>(

  (

    `test`.`t1`.`a1` > (/* select#2 */

      select min(`test`.`t2`.`a2`)

      from `test`.`t2`

      where (`test`.`t2`.`a2` > 10)

    )

  )

)

從查詢執(zhí)行計劃看，出現(xiàn)了子查詢（SUBQUERY），但是，子查詢被“min”函數(shù)限制，而子查詢中的被查詢列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“>ANY”式的子查詢優(yōu)化，子查詢只被執(zhí)行一次即可求得最小值。

 

使用了“=ANY”式子的子查詢被優(yōu)化，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 =ANY (SELECT a2 FROM t2 WHERE t2.a2>10);


+----+--------------+-------------+------+------+----------------------------------------------------+

| id | select_type | table    | type | key | Extra  |

+----+--------------+-------------+------+------+----------------------------------------------------+

| 1 | SIMPLE    | <subquery2> | ALL | NULL | NULL  |

| 1 | SIMPLE    | t1     | ALL | NULL | Using where; Using join buffer (Block Nested Loop) |

| 2 | MATERIALIZED | t2     | ALL | NULL | Using where  |

+----+--------------+-------------+------+------+----------------------------------------------------+

3 rows in set, 1 warning (0.02 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1` semi join (`test`.`t2`)

where ((`test`.`t1`.`a1` = `<subquery2>`.`a2`) and (`test`.`t2`.`a2` > 10))

從查詢執(zhí)行計劃看，沒有出現(xiàn)了子查詢，表t2被物化，與表t1進行了半連接。

 

使用了“<ANY”式子的子查詢被優(yōu)化，查詢執(zhí)行計劃如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 <ANY (SELECT a2 FROM t2 WHERE t2.a2>10);


+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)

被查詢優(yōu)化器處理后的語句為：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where <nop>(

  (

    `test`.`t1`.`a1` < (/* select#2 */

      select max(`test`.`t2`.`a2`)

      from `test`.`t2`

      where (`test`.`t2`.`a2` > 10)

    )

  )

)

從查詢執(zhí)行計劃看，出現(xiàn)了子查詢（SUBQUERY），但是，子查詢被“max”函數(shù)限制，而子查詢中的被查詢列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“<ANY”式的子查詢優(yōu)化，子查詢只被執(zhí)行一次即可求得最大值。

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