MySQL Forums
Forum List  »  Optimizer & Parser

Query not using index sporadically
Posted by: Vinay NP
Date: June 13, 2014 07:51AM

We have a query which runs on a table with ~50M rows. The query does not use the correct index sometimes. This happens only once in a while and we have so far been unsuccessful in finding a pattern. Whenever this does happen, more queries start piling up and the entire db becomes sluggish.

I've illustrated the query and the table structure (similar to original) below.

Table 1 (~50M rows) Structure
-----------------
CREATE TABLE IF NOT EXISTS `rluf` (
`id` int(11) NOT NULL,
`group_id` varchar(13) NOT NULL,
`email` varchar(100) NOT NULL,
`mobile` varchar(50) NOT NULL,
`name` varchar(200) NOT NULL,
PRIMARY KEY (`id`),
KEY `group_id` (`group_id`),
KEY `email` (`email`),
KEY `mobile` (`mobile`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Table 2 Structure
-----------------
CREATE TABLE IF NOT EXISTS `rlbl` (
`id` int(11) NOT NULL,
`group_id` varchar(13) NOT NULL,
`user_id` varchar(100) NOT NULL,
`status` varchar(100) NOT NULL,
`type` varchar(50) NOT NULL
PRIMARY KEY (`id`),
KEY `group_id` (`group_id`),
KEY `user_id` (`user_id`),
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Query
-----
SELECT DISTINCT
rlbl.group_id,
rlbl.status,
rluf.email,
rluf.mobile,
rluf.name,
rlbl.type
FROM
`rlbl`
RIGHT JOIN
`rluf` ON ( rlbl.group_id = rluf.group_id )
WHERE
`rlbl`.`user_id` IN (
'kjsdhjs5354bkdf',
'kjbsdkbsd232323'
)
;

This query works fine when it uses the intended indexes rlbl.user_id and rluf.group_id. However, at times it does not use rluf.group_id and then the problem described above happens.

Some observations
1. Checked query plan for key usage other times -- indexes are used as intended
2. Checked query plan for key usage during issue -- indexes are not used as intended. rluf.group_id index not used resulting in full table scan.
2. Checked any deadlocks during failure -- No deadlocks
3. Forced index -- Not effective

MySQL version: 5.1.52-log

Any help regarding why this issue might be occuring and what could be the possible resolution is appreciated.

Options: ReplyQuote


Subject
Views
Written By
Posted
Query not using index sporadically
2458
June 13, 2014 07:51AM


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.