MySql不与ORDER BY一起使用索引

Question

I have a table with this structure (with 50k fields):

CREATE TABLE IF NOT EXISTS `comments` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `imageid` int(10) unsigned NOT NULL DEFAULT '0',
  `uid` bigint(20) unsigned NOT NULL DEFAULT '0',
  `content` text CHARACTER SET utf8,
  `adate` datetime DEFAULT NULL,
  `ip` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `ids` (`imageid`,`adate`) USING BTREE
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=52236 ;

I want to select data by imageid and sort it with adate so I added ( imageid , adate ) key.

but the explain result of this query says MuSQL still use table scan. Why?!

EXPLAIN SELECT   comments.*
FROM comments
WHERE comments.imageid=50
ORDER BY
comments.adate DESC LIMIT 10

result:

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   SIMPLE  comments    ref     ids     ids     4   const   203     Using where

and with this index:

KEY `ids` (`imageid`,`adate`,`id`) USING BTREE

the result for this query:

EXPLAIN SELECT   comments.id
FROM comments
WHERE comments.imageid=50
ORDER BY
comments.adate DESC LIMIT 10

IS:

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   SIMPLE  comments    ref     ids     ids     4   const   203     Using where; Using index

Answer 1

You are not reading the results from explain correctly.

using index means that the query is index-covered - the data is read only from the index, and the actual row is not used. This is not because the index is different, but because in the second query you are selecring only the id.

If MySQL does not use index for resolving the ORDER BY there would be using filesort in the explain. In both queries, ids key is used and MySQL does not perofrm a table scan

Answer 2

For InnoDB create a key like this (imageid,adate,id); order of column in index is important.

Then try this -

EXPLAIN SELECT   comments.id
FROM comments
WHERE comments.imageid=50
ORDER BY
comments.adate DESC LIMIT 10

Let me know what is the output.

Thanks for the output. Adding following section below.

If you require more than id then try something like this

EXPLAIN 
SELECT c1.* from comments as c1
JOIN
(
   SELECT comments.id
   FROM comments
   WHERE comments.imageid=50
   ORDER BY
   comments.adate DESC LIMIT 10
) as c2 ON (c1.id=c2.id)

But this time do not rely on Explain. Explain will show one row with index anyway still. Rather check execution time in phpmyadmin or in mysql query browser.

Answer 3

Straight out of the manual:

In some cases, MySQL cannot use indexes to resolve the ORDER BY, although it still uses indexes to find the rows that match the WHERE clause. These cases include the following: ––– You use ORDER BY on nonconsecutive parts of a key: SELECT * FROM t1 WHERE key2=constant ORDER BY key_part2;

Maybe this applies to your query since you have a composite KEY ids (imageid,adate) and you don't use imageid in your ORDER BY . You could try and add a key for adate only and see if that helps.