index merge合併索引

raysuen發表於2017-04-24
Index索引

mysql> create index ind_t2_name on t2(name);
Query OK, 0 rows affected (0.18 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> create index ind_t2_addr on t2(address);
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select * from t2 where name='name44' or address='address888';
+----+-------------+-------+------------+-------------+-------------------------+-------------------------+---------+------+------+----------+---------------------------------------------------+
| id | select_type | table | partitions | type        | possible_keys           | key                     | key_len | ref  | rows | filtered | Extra                                             |
+----+-------------+-------+------------+-------------+-------------------------+-------------------------+---------+------+------+----------+---------------------------------------------------+
|  1 | SIMPLE      | t2    | NULL       | index_merge | ind_t2_name,ind_t2_addr | ind_t2_name,ind_t2_addr | 63,153  | NULL |    2 |   100.00 | Using union(ind_t2_name,ind_t2_addr); Using where |
+----+-------------+-------+------------+-------------+-------------------------+-------------------------+---------+------+------+----------+---------------------------------------------------+
1 row in set, 1 warning (0.53 sec)


在mysql5.5或以前的版本單表只能使用一條索引，不能對2條索引進行合併，最好還是把or修改成union all


select * from t2 where name='name44’ union all select * from t2 where address='address888'; 

來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/28572479/viewspace-2137876/，如需轉載，請註明出處，否則將追究法律責任。

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