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query scanning all rows
Posted by: jay oberoi
Date: September 12, 2012 01:20PM

Guys trying to wrap my head around mysql query on why it is scanning all rows in table

I have 2 tables topic_entry and topic_user

CREATE TABLE `topic_entry` (
`entry_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`topic_id` bigint(20) unsigned NOT NULL,
`entry_created` datetime NOT NULL,
`entry_object` text,
`level` tinyint(4) NOT NULL DEFAULT '3',
PRIMARY KEY (`entry_id`),
KEY `entry_created` (`entry_created`),
KEY `level` (`level`),
KEY `topic_id_2` (`topic_id`,`entry_id`)
) ENGINE=MyISAM;

CREATE TABLE `topic_user` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`topic_id` bigint(20) unsigned NOT NULL,
`user_id` varchar(100) NOT NULL,
`private` enum('y','n') DEFAULT NULL,
`freq` enum('a','d','w') DEFAULT NULL,
`topic_id_freq` varchar(10) DEFAULT NULL,
`casematch` enum('0','1') DEFAULT '0',
`textmatch` enum('0','1') DEFAULT '0',
`topic_name_case` varchar(100) DEFAULT '',
PRIMARY KEY (`id`),
UNIQUE KEY `topic_id_user_id` (`topic_id`,`user_id`),
KEY `user_id` (`user_id`),
KEY `topic_id_freq` (`topic_id_freq`)
) ENGINE=MyISAM;
This is the query that i am trying to run

explain
select te.topic_id,te.entry_id
from topic_entry te
WHERE te.topic_id in (select topic_id
from topic_user where user_id ='xyz')
AND te.level=4

ORDER BY te.entry_id DESC
LIMIT 5;


The explain output shows that it is scanning all rows
| 1 | PRIMARY| te | range | PRIMARY,level | PRIMARY | 8 | NULL| **722978** | Using where |
| 2 | DEPENDENT SUBQUERY | topic_user | unique_subquery | topic_id_user_id,user_id | topic_id_user_id | 310 | func,const | 1 | Using index; Using where |

If i remove order by clause from query it work perfectly
Any pointers would be appreciated

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