Hello.




MySQL uses indexes in queries which are parts of UNION. See:



mysql> explain (select * from t1 where a=5) union (select * from t1 where 
a=3)\G;

*************************** 1. row ***************************

           id: 1

  select_type: PRIMARY

        table: t1

         type: ref

possible_keys: a

          key: a

      key_len: 5

          ref: const

         rows: 1

        Extra: Using where; Using index

*************************** 2. row ***************************

           id: 2

  select_type: UNION

        table: t1

         type: ref

possible_keys: a

          key: a

      key_len: 5

          ref: const

         rows: 1

        Extra: Using where; Using index

*************************** 3. row ***************************

           id: NULL

  select_type: UNION RESULT

        table: <union1,2>

         type: ALL

possible_keys: NULL

          key: NULL

      key_len: NULL

          ref: NULL

         rows: NULL

        Extra:

3 rows in set (0.00 sec)





mysql> show create table t1\G;

*************************** 1. row ***************************

       Table: t1

Create Table: CREATE TABLE `t1` (

  `a` int(11) default NULL,

  KEY `a` (`a`)

) ENGINE=MyISAM DEFAULT CHARSET=latin1







> Hello,

> 

> MySQL implemented index_merge in version 5...

> Does MySQL supports something like index_merge to speed up UNION 

> queries? If yes, for which version (assumed release time)?

> Does anyone know of other DBs systems that can speed up UNION queries?

> This issue is quite critical for our system..

> 

> -thanks in advance.

> 

> 

> 



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