I’m trying to select the time field from five tables and return the oldest time. The userid field in every table is indexed. Here’s the query:

(
SELECT  `time` 
FROM  `eventlog` 
WHERE userid =  '9819'
)
UNION ALL (

SELECT  `time` 
FROM  `screenshots` 
WHERE userid =  '9819'
)
UNION ALL (

SELECT  `time` 
FROM  `sitelog` 
WHERE userid =  '9819'
)
UNION ALL (

SELECT  `time` 
FROM  `keylog` 
WHERE userid =  '9819'
)
UNION ALL (

SELECT  `time` 
FROM  `webcamcaps` 
WHERE userid =  '9819'
)
ORDER BY  `time` ASC 
LIMIT 1;

Here’s the EXPLAIN:

+----+--------------+------------------+------+---------------+--------+---------+-------+------+----------------+
| id | select_type  | table            | type | possible_keys | key    | key_len | ref   | rows | Extra          |
+----+--------------+------------------+------+---------------+--------+---------+-------+------+----------------+
|  1 | PRIMARY      | eventlog         | ref  | userid        | userid | 4       | const |  308 |                |
|  2 | UNION        | screenshots      | ref  | userid        | userid | 4       | const | 7850 |                |
|  3 | UNION        | sitelog          | ref  | userid        | userid | 4       | const | 1949 |                |
|  4 | UNION        | keylog           | ref  | userid        | userid | 4       | const | 3589 |                |
|  5 | UNION        | webcamcaps       | ref  | userid        | userid | 4       | const | 8035 |                |
| NULL | UNION RESULT | <union1,2,3,4,5> | ALL  | NULL          | NULL   | NULL    | NULL  | NULL | Using filesort |
+----+--------------+------------------+------+---------------+--------+---------+-------+------+----------------+
6 rows in set (0.00 sec)

How can I optimize this so I’m not looking through so many rows?

Take advantage of the index for each table doing something like this:

(SELECT  MIN(`time`) `time` FROM  `eventlog` WHERE userid =  '9819') UNION ALL
(SELECT  MIN(`time`) FROM  `screenshots` WHERE userid =  '9819')     UNION ALL
(SELECT  MIN(`time`) FROM  `sitelog` WHERE userid =  '9819')         UNION ALL
(SELECT  MIN(`time`) FROM  `keylog` WHERE userid =  '9819')          UNION ALL
(SELECT  MIN(`time`) FROM  `webcamcaps` WHERE userid =  '9819')
ORDER BY  `time` ASC 
LIMIT 1;

or

(SELECT  `time` FROM  `eventlog` WHERE userid =  '9819' ORDER BY time ASC LIMIT 1)
UNION ALL
(SELECT  `time` FROM  `screenshots` WHERE userid =  '9819' ORDER BY time ASC LIMIT 1)
UNION ALL
(SELECT  `time` FROM  `sitelog` WHERE userid =  '9819' ORDER BY time ASC LIMIT 1)
UNION ALL
(SELECT  `time` FROM  `keylog` WHERE userid =  '9819' ORDER BY time ASC LIMIT 1)
UNION ALL
(SELECT  `time` FROM  `webcamcaps` WHERE userid =  '9819' ORDER BY time ASC LIMIT 1)
ORDER BY  `time  ASC 
LIMIT 1;

The main idea is to get each subquery to return 1 row.

Give it a Try !!!