本文基于MySQL5.7.19测试
创建四张表，pt1、pt2表加上主键
mysql> create table t1 (a1 int, b1 int);
mysql> create table t2 (a2 int, b2 int);
mysql> create table pt1 (a1 int, b1 int, primary key (a1));
mysql> create table pt2 (a2 int, b2 int, primary key (a2));
向表中分别插入10000条数据
mysql> delimiter //
mysql> create procedure prod_dt()
-> begin
-> declare i int;
-> set i=0;
-> while i<10000 do
-> insert into t1(a1,b1) values(i,i+1);
-> insert into t2(a2,b2) values(i+1,i+2);
-> insert into pt1(a1,b1) values(i,i+1);
-> insert into pt2(a2,b2) values(i+1,i+2);
-> set i=i+1;
-> end while;
-> end;
-> //
Query OK, 0 rows affected (0.05 sec) mysql> delimiter ;
mysql> call prod_dt() ;
MySQL支持对简单SELECT查询中的子查询优化，包括：
1 简单SELECT查询中的子查询。
2 带有DISTINCT、ORDERBY、LIMIT操作的简单SELECT查询中的子查询。
# 没有主键，优化器进行了优化，子查询物化后和表t1进行连接。执行计划中没有子查询
mysql> explain extended select * from t1 where t1.a1<100 and a1 in (select a2 from t2 where t2.a2 >10);
+----+--------------+-------------+------------+------+---------------+------+---------+------+-------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+-------------+------------+------+---------------+------+---------+------+-------+----------+----------------------------------------------------+
| 1 | SIMPLE | <subquery2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | 100.00 | Using where |
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10362 | 3.33 | Using where; Using join buffer (Block Nested Loop) |
| 2 | MATERIALIZED | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10157 | 33.33 | Using where |
+----+--------------+-------------+------------+------+---------------+------+---------+------+-------+----------+----------------------------------------------------+
3 rows in set, 2 warnings (0.00 sec) #有主键，优化器进行了优化。执行计划中没有子查询
mysql> explain extended select * from pt1 where pt1.a1<100 and a1 in (select a2 from pt2 where pt2.a2 >10);
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------+------+----------+-------------+
| 1 | SIMPLE | pt1 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 89 | 100.00 | Using where |
| 1 | SIMPLE | pt2 | NULL | eq_ref | PRIMARY | PRIMARY | 4 | abce.pt1.a1 | 1 | 100.00 | Using index |
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec) mysql>
MySQL不支持对如下情况的子查询进行优化：
-带有UNION操作。
-带有GROUPBY、HAVING、聚集函数。
-使用ORDERBY中带有LIMIT。
-内表、外表的个数超过MySQL支持的最大表的连接数。
#有聚合函数，没有进行子查询优化
mysql> explain extended select * from t1 where t1.a1>(select min(t2.a2) from t2);
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | PRIMARY | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10362 | 33.33 | Using where |
| 2 | SUBQUERY | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10157 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
2 rows in set, 2 warnings (0.01 sec) mysql> explain extended select * from pt1 where pt1.a1>(select min(pt2.a2) from pt2);
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+------------------------------+
| 1 | PRIMARY | pt1 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 5000 | 100.00 | Using where |
| 2 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+------------------------------+
2 rows in set, 2 warnings (0.00 sec) mysql>
MySQL支持哪些子查询的优化技术？
1 子查询合并技术 --> 不支持
#t2表上执行了2次子查询。如果支持子查询合并技术,则t2表上只执行一次子查询
mysql> explain extended select * from t1 where a1<4 and (exists (select a2 from t2 where t2.a2<5 and t2.b2=1) or exists (select a2 from t2 where t2.a2<5 and t2.b2=2) );
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | PRIMARY | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10362 | 33.33 | Using where |
| 3 | SUBQUERY | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10157 | 3.33 | Using where |
| 2 | SUBQUERY | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10157 | 3.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
3 rows in set, 2 warnings (0.00 sec) mysql> #pt2表上执行了2次子查询。如果支持子查询合并技术,则pt2表上只执行一次子查询
mysql> explain extended select * from pt1 where a1<4 and (exists (select a2 from pt2 where pt2.a2<5 and pt2.b2=1) or exists (select a2 from pt2 where pt2.a2<5 and pt2.b2=2) );
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | PRIMARY | pt1 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 4 | 100.00 | Using where |
| 3 | SUBQUERY | pt2 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 4 | 10.00 | Using where |
| 2 | SUBQUERY | pt2 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 4 | 10.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
3 rows in set, 2 warnings (0.00 sec) mysql>
#人为的合并查询条件为“(t2.b2=1 OR t2.b2=2)”t2表上的子查询，只执行一次
mysql> explain extended select * from t1 where a1<10 and exists (select a2 from t2 where t2.a2<5 and (t2.b2=1 or t2.b2=2));
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | PRIMARY | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10362 | 33.33 | Using where |
| 2 | SUBQUERY | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10157 | 6.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
2 rows in set, 2 warnings (0.01 sec) mysql> explain extended select * from pt1 where a1<10 and exists (select a2 from pt2 where pt2.a2<5 and (pt2.b2=1 or pt2.b2=2));
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | PRIMARY | pt1 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 10 | 100.00 | Using where |
| 2 | SUBQUERY | pt2 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 4 | 19.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec) mysql>
2 子查询展开(子查询反嵌套)技术 --> 支持得不够好
mysql> explain extended select * from t1, (select * from t2 where t2.a2 >10) v_t2 where t1.a1<10 and v_t2.a2<20;
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+----------------------------------------------------+
| 1 | SIMPLE | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10157 | 11.11 | Using where |
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10362 | 33.33 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+----------------------------------------------------+
2 rows in set, 2 warnings (0.00 sec) mysql> explain extended select * from pt1, (select * from pt2 where pt2.a2 >10) v_t2 where pt1.a1<10 and v_t2.a2<20;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | pt2 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 9 | 100.00 | Using where |
| 1 | SIMPLE | pt1 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 10 | 100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+----------------------------------------------------+
2 rows in set, 2 warnings (0.00 sec) mysql> #IN子查询的例子,可以看出子查询被物化
mysql> explain extended select * from t1 where t1.a1<100 and a1 in (select a2 from t2 where t2.a2 >10);
+----+--------------+-------------+------------+------+---------------+------+---------+------+-------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+-------------+------------+------+---------------+------+---------+------+-------+----------+----------------------------------------------------+
| 1 | SIMPLE | <subquery2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | 100.00 | Using where |
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10362 | 3.33 | Using where; Using join buffer (Block Nested Loop) |
| 2 | MATERIALIZED | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10157 | 33.33 | Using where |
+----+--------------+-------------+------------+------+---------------+------+---------+------+-------+----------+----------------------------------------------------+
3 rows in set, 2 warnings (0.00 sec) mysql> #从查询执行计划看，子查询不存在，SQL语句被转换为内连接操作，这表明MySQL只有在针对主键列进行类似的子查询时，才把子查询上拉为内连接。所以，MySQL还是支持子查询展开技术的。
mysql> explain extended select * from pt1 where pt1.a1<100 and a1 in (select a2 from pt2 where pt2.a2 >10);
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------+------+----------+-------------+
| 1 | SIMPLE | pt1 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 89 | 100.00 | Using where |
| 1 | SIMPLE | pt2 | NULL | eq_ref | PRIMARY | PRIMARY | 4 | abce.pt1.a1 | 1 | 100.00 | Using index |
+----+-------------+-------+------------+--------+---------------+---------+---------+-------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec) mysql>
3 聚集子查询消除技术 --> 不支持
#MySQL认为，聚集子查询，只需要执行一次，得到结果后，即可把结果缓冲到内存*后续连接或过滤等操作使用，没有必要消除掉子查询。
#另外，如果聚集子查询在索引列上执行，则会更快得到查询结果，更加能加速查询速度。
mysql> explain extended select * from t1 where t1.a1>(select min(t2.a2) from t2);
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | PRIMARY | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 10362 | 33.33 | Using where |
| 2 | SUBQUERY | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10157 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec) mysql> explain extended select * from pt1 where pt1.a1>(select min(pt2.a2) from pt2);
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+------------------------------+
| 1 | PRIMARY | pt1 | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 5000 | 100.00 | Using where |
| 2 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+------------------------------+
2 rows in set, 2 warnings (0.00 sec) mysql>