MySQL: Why is this SQL-query not using index?

Question

I have a very simple SELECT that resorts to filesort and does not use index.

Consider the following query:

SELECT * FROM forum_topic
WHERE topic_status = 0
ORDER BY modified_date LIMIT 0, 30

on the following table ( stripped of a few columns to make it more brief here )

CREATE TABLE `forum_topic` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`slug` varchar(255) NOT NULL,
`forum_id` int(10) NOT NULL DEFAULT '1',
`title` varchar(100) NOT NULL,
`topic_status` tinyint(1) NOT NULL DEFAULT '0',
`post_count` bigint(20) NOT NULL DEFAULT '0',
`modified_date` datetime NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `slug` (`slug`),
  FULLTEXT KEY `title` (`title`),
  KEY `modified` (`modified_date`, `topic_status`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

EXPLAIN gives the following output

id  select_type table?      partitions? type?   possible_keys?  key?    key_len?    ref?    rows?   Extra?
1   SIMPLE      forum_topic NULL        ALL     NULL            NULL    NULL    NULL    2075    Using where; Using filesort

Notice how the explain says there are NULL for possible_keys and how it's using filesort after having scanned ALL rows.

Please advice. Thanks.

Answer 1

This query needs topic_status to appear in the most significant position of an index, because it's searching on a constant.

You have

   KEY `modified` (`modified_date`, `topic_status`)

and you may want

   KEY `mod2` (`topic_status`, `modified_date` )

instead. This may satisfy both the filter and the ORDER BY ... LIMIT part of the query.

Pro tip: Avoid SELECT * and enumerate the columns you actually need instead.

Pro tip: Filesort doesn't necessarily mean what you think it means. It's used anytime MySQL needs to construct an intermediate result set for such things as sorting.