MySQL Server version: 5.0.95
Tables All: InnoDB
I am having an issue with a MySQL db query. Basically I am finding that if I index a particular varchar(50) field tag.name, my queries take longer (x10) than not indexing the field. I am trying to speed this query up, however my efforts seem to be counter productive.
The culprit line and field seems to be:
WHERE `t`.`name` IN ('news','home')
I have noticed that if i query the tag table directly without a join using the same criteria and with the name field indexed, i do not have the issue.. It actually works faster as expected.
EXAMPLE Query **
SELECT `a`.*, `u`.`pen_name`
FROM `tag_link` `tl`
INNER JOIN `tag` `t`
ON `t`.`tag_id` = `tl`.`tag_id`
INNER JOIN `article` `a`
ON `a`.`article_id` = `tl`.`link_id`
INNER JOIN `user` `u`
ON `a`.`user_id` = `u`.`user_id`
WHERE `t`.`name` IN ('news','home')
AND `tl`.`type` = 'article'
AND `a`.`featured` = 'featured'
GROUP BY `article_id`
LIMIT 0 , 5
EXPLAIN with index **
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+--------------------------+---------+---------+-------------------+------+-----------------------------------------------------------+
| 1 | SIMPLE | t | range | PRIMARY,name | name | 152 | NULL | 4 | Using where; Using index; Using temporary; Using filesort |
| 1 | SIMPLE | tl | ref | tag_id,link_id,link_id_2 | tag_id | 4 | portal.t.tag_id | 10 | Using where |
| 1 | SIMPLE | a | eq_ref | PRIMARY,fk_article_user1 | PRIMARY | 4 | portal.tl.link_id | 1 | Using where |
| 1 | SIMPLE | u | eq_ref | PRIMARY | PRIMARY | 4 | portal.a.user_id | 1 | |
+----+-------------+-------+--------+--------------------------+---------+---------+-------------------+------+-----------------------------------------------------------+
EXPLAIN without index **
+----+-------------+-------+--------+--------------------------+---------+---------+---------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+--------------------------+---------+---------+---------------------+------+-------------+
| 1 | SIMPLE | a | index | PRIMARY,fk_article_user1 | PRIMARY | 4 | NULL | 8742 | Using where |
| 1 | SIMPLE | u | eq_ref | PRIMARY | PRIMARY | 4 | portal.a.user_id | 1 | |
| 1 | SIMPLE | tl | ref | tag_id,link_id,link_id_2 | link_id | 4 | portal.a.article_id | 3 | Using where |
| 1 | SIMPLE | t | eq_ref | PRIMARY | PRIMARY | 4 | portal.tl.tag_id | 1 | Using where |
+----+-------------+-------+--------+--------------------------+---------+---------+---------------------+------+-------------+
TABLE CREATE
CREATE TABLE `tag` (
`tag_id` int(11) NOT NULL auto_increment,
`name` varchar(50) NOT NULL,
`type` enum('layout','image') NOT NULL,
`create_dttm` datetime default NULL,
PRIMARY KEY (`tag_id`)
) ENGINE=InnoDB AUTO_INCREMENT=43077 DEFAULT CHARSET=utf8
INDEXS
SHOW INDEX FROM tag_link;
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| tag_link | 0 | PRIMARY | 1 | tag_link_id | A | 42023 | NULL | NULL | | BTREE | |
| tag_link | 1 | tag_id | 1 | tag_id | A | 10505 | NULL | NULL | | BTREE | |
| tag_link | 1 | link_id | 1 | link_id | A | 14007 | NULL | NULL | | BTREE | |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
SHOW INDEX FROM article;
+---------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| article | 0 | PRIMARY | 1 | article_id | A | 5723 | NULL | NULL | | BTREE | |
| article | 1 | fk_article_user1 | 1 | user_id | A | 1 | NULL | NULL | | BTREE | |
| article | 1 | create_dttm | 1 | create_dttm | A | 5723 | NULL | NULL | YES | BTREE | |
+---------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
Final Solution
It seems that MySQL is just sorted the data incorrectly. In the end it turned out faster to look at the tag table as a sub query returning the ids.
How big are your tables?
I noticed in the first explain you have a “Using temporary; Using filesort” which is bad. Your query is likely being dumped to disc which makes it way slower than in memory queries.
Also try to avoid using “select *” and instead query the minimum fields needed.