Mysql 5.6优化器没有';不要在小表联接中使用索引
我们有两个表-第一个相对较大(联系人表)250k行,第二个较小(用户表,<10行)。在mysql 5.6版本上,我有下一个解释结果:Mysql 5.6优化器没有';不要在小表联接中使用索引,mysql,Mysql,我们有两个表-第一个相对较大(联系人表)250k行,第二个较小(用户表,
EXPLAIN SELECT
o0_.id AS id_0,
o8_.first_name,
o8_.last_name
FROM
contact o0_
LEFT JOIN user o8_ ON o0_.user_owner_id = o8_.id
LIMIT
25 OFFSET 100
+----+-------------+-------+-------+---------------+----------------------+---------+------+--------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------------------+---------+------+--------+----------------------------------------------------+
| 1 | SIMPLE | o0_ | index | NULL | IDX_403263ED9EB185F9 | 5 | NULL | 253030 | Using index |
| 1 | SIMPLE | o8_ | ALL | PRIMARY | NULL | NULL | NULL | 5 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+---------------+----------------------+---------+------+--------+----------------------------------------------------+
一组2行(0.00秒)
当我对联接使用强制索引时:
EXPLAIN SELECT
o0_.id AS id_0,
o8_.first_name,
o8_.last_name
FROM
contact o0_
LEFT JOIN user o8_ force index for join(`PRIMARY`) ON o0_.user_owner_id = o8_.id
LIMIT
25 OFFSET 100
或者在用户表的select子句(first_name,last_name)中显示的字段上添加索引:
alter table user add index(first_name, last_name);
解释对此的结果更改:
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
| 1 | SIMPLE | o0_ | index | NULL | IDX_403263ED9EB185F9 | 5 | NULL | 253030 | Using index |
| 1 | SIMPLE | o8_ | eq_ref | PRIMARY | PRIMARY | 4 | o0_.user_owner_id | 1 | NULL |
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
2 rows in set (0,00 sec)
在mysql 5.5版本上,我有相同的解释结果,没有附加索引:
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
| 1 | SIMPLE | o0_ | index | NULL | IDX_403263ED9EB185F9 | 5 | NULL | 255706 | Using index |
| 1 | SIMPLE | o8_ | eq_ref | PRIMARY | PRIMARY | 4 | o0_.user_owner_id | 1 | |
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
2 rows in set (0.00 sec)
为什么我需要在mysql 5.6版本上强制使用主索引或添加额外索引?
当连接小表时,其他选择也会发生同样的行为。如果表中的行数太少,那么执行完整表扫描实际上可能比执行索引、查找记录然后返回表更快。如果在用户表中除了查询中的3之外还有其他字段,则可以考虑添加覆盖索引,但是Fravy,我认为这些都不会对查询速度产生显著影响。问题是,这些连接与完整表扫描的性能影响是显著的:索引为0.04秒,而完整表扫描为3.61秒,然后保留索引提示。