如何理解index merge合并索引

mysql> create index ind_t2_name on t2(name);

Query OK, 0 rows affected (0.18 sec)

Records: 0Duplicates: 0Warnings: 0

mysql> create index ind_t2_addr on t2(address);

Query OK, 0 rows affected (0.11 sec)

Records: 0Duplicates: 0Warnings: 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或以前的版本单表只能使用一条索引，不能对2条索引进行合并，最好还是把or修改成union all

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