模式匹配like'%XXX%'优化
在MySQL里，like'XXX%可以用到索引，但like '%XXX%'却不行，比如，以下这个案例：
查看测试表行数：
两次like匹配对比：
优化思路：
这个测试表中，id是主键，叶子节点上保存了数据，从索引中就可以去到select的的id的列，不必读取数据行（只有select字段正好就是索引，那么就用到了覆盖索引），通过覆盖索引，减少I/O，提高性能。
优化之前的执行计划：
优化之后的执行计划：
查看测试表行数：
点击(此处)折叠或打开
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mysql> select count(*) from test03;
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+----------+
-
| count(*) |
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+----------+
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| 117584 |
- +----------+
点击(此处)折叠或打开
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mysql> explain select count(*) from test03 where username like '1%';
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+----+-------------+--------+-------+-----------------+-----------------+---------+------+-------+--------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+--------+-------+-----------------+-----------------+---------+------+-------+--------------------------+
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| 1 | SIMPLE | test03 | range | idx_test03_name | idx_test03_name | 302 | NULL | 58250 | Using where; Using index |
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+----+-------------+--------+-------+-----------------+-----------------+---------+------+-------+--------------------------+
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1 row in set (0.03 sec)
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mysql> explain select count(*) from test03 where username like '%1%';
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+----+-------------+--------+-------+---------------+-----------------+---------+------+--------+--------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+--------+-------+---------------+-----------------+---------+------+--------+--------------------------+
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| 1 | SIMPLE | test03| index | NULL | idx_test03_name | 302 | NULL | 116500 | Using where; Using index |
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+----+-------------+--------+-------+---------------+-----------------+---------+------+--------+--------------------------+
- 1 row in set (0.00 sec)
这个测试表中，id是主键，叶子节点上保存了数据，从索引中就可以去到select的的id的列，不必读取数据行（只有select字段正好就是索引，那么就用到了覆盖索引），通过覆盖索引，减少I/O，提高性能。
优化之前的执行计划：
点击(此处)折叠或打开
-
mysql> explain select count(*) from test03 where username like '%1%';
-
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
-
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
-
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
-
| 1 | SIMPLE | test03 | ALL | NULL | NULL | NULL | NULL | 7164 | Using where |
- +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
点击(此处)折叠或打开
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mysql> explain select count(*) from test03 a join (select id from test03 where username like '%1%') b on a.id=b.id;
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+----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+
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| 1 | PRIMAR | <derived2> | ALL | NULL | NULL | NULL | NULL | 7164 | NULL |
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| 1 | PRIMARY | a | eq_ref | PRIMARY | PRIMARY | 8 | b.id | 1 | Using index |
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| 2 | DERIVED | test03 | ALL | NULL | NULL | NULL | NULL | 7164 | Using where |
- +----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+