[英]Mysql 5.6 optimizer doesn't use indexes in small tables joins
我们有两个表 - 第一个是相对较大的(联系表)250k行,第二个是小表(用户表,<10行)。 在mysql 5.6版本上我有下一个解释结果:
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) |
+----+-------------+-------+-------+---------------+----------------------+---------+------+--------+----------------------------------------------------+
2行(0,00秒)
当我使用force index进行连接时:
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
或在用户表上的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);
解释结果更改为:
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
| 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)
为什么我需要强制使用PRIMARY索引或在mysql 5.6版本上添加额外的索引? 当连接小表时,其他选择会发生相同的行为。
如果你有一个行数这么少的表,那么进行全表扫描实际上要比转到索引更快,找到记录然后再回到表中。 如果除了查询中的3之外,用户表中还有其他字段,那么您可以考虑添加覆盖索引,但坦率地说,我认为这不会对查询的速度产生重大影响。
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