multiple-column index and order by
I'm confused --- Why can't MySQL use the index in the following example?
I am assuming that the tuples are physically stored by the multiple-column primary key.
So why can't it jump to t.col1 = 16, and then simply grab the next 100 tuples?
Instead, it is reading all tuples with t.col1 = 16 and then sorting them?!?
CREATE TABLE `mytable` (
`col1` smallint(6) NOT NULL,
`col2` int(11) NOT NULL,
`col3` tinyint(4) NOT NULL,
`col4` tinyint(4) NOT NULL,
PRIMARY KEY (`col1`,`col2`,`col3`,`col4`)
) ENGINE=MyISAM;
mysql> explain SELECT * FROM mytable t WHERE t.col1 = 16 ORDER BY t.col2 LIMIT 100;
+----+-------------+-------+------+---------------+---------+---------+-------+---------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+---------+---------+-------+---------+--------------------------+
| 1 | SIMPLE | di2b | ref | PRIMARY | PRIMARY | 2 | const | 5190118 | Using where; Using index |
+----+-------------+-------+------+---------------+---------+---------+-------+---------+--------------------------+
Thanks.
Raphael
Subject
Views
Written By
Posted
multiple-column index and order by
3114
July 06, 2012 05:59PM
1496
July 10, 2012 12:55PM
Sorry, you can't reply to this topic. It has been closed.
Content reproduced on this site is the property of the respective copyright holders.
It is not reviewed in advance by Oracle and does not necessarily represent the opinion
of Oracle or any other party.