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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