Question

I have a large mysql table (about 5M rows) on which i frequently insert data.

This table is the same i have to read data from and sometimes the entire database gets slow because of selecting data while there are many pending inserts.

I put indexes on each field i use in the WHERE statment, so i really don’t know why select gets so slow.

Could anyone provide me a hint to solve this problem ?

here is the sql of table and query

CREATE TABLE `messages` (
  `id` int(10) unsigned NOT NULL auto_increment,
  `user_id` int(10) unsigned NOT NULL default '0',
  `dest` varchar(20) character set latin1 default NULL,
  `body` text character set latin1,
  `sent_on` timestamp NOT NULL default CURRENT_TIMESTAMP,
  `md5` varchar(32) character set latin1 NOT NULL default '',
  `interface` enum('mobile','desktop') default NULL,
  PRIMARY KEY  (`id`),
  KEY `user_id` (`user_id`),
  KEY `md5` (`md5`),
  FULLTEXT KEY `dest` (`dest`,`body`),
  FULLTEXT KEY `body` (`body`)
) ENGINE=MyISAM AUTO_INCREMENT=7074256 DEFAULT CHARSET=utf8

and here the query:

EXPLAIN SELECT SQL_CALC_FOUND_ROWS id, sent_on, dest AS who, body,interface FROM messages WHERE user_id = 2 ORDER BY sent_on DESC LIMIT 0,50 \G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: messages
         type: ref
possible_keys: user_id
          key: user_id
      key_len: 4
          ref: const
         rows: 13997
        Extra: Using where; Using filesort
1 row in set (0.00 sec)

Answer 1

Note the following in your EXPLAIN output:

Extra: Using where; Using filesort

The Using filesort means that MySQL is dumping the query results to a file to sort it, then reading the results back in to get the top 50 rows.

While I’m no expert, I think that you could optimize this process by providing an index which can both satisfy the selection criteria and sort order all in one go; then the selection and ordering can be determiend by an index scan only, without having to sort the result set every time.

In this case, your WHERE is on user_id, and your ORDER BY is on sent_on. So, in theory, if you provide a single index on those two columns (in that order), then the engine will be able to use the first half of the index to filter the results, and because the second half of the index is on the sent_on column, the index results will already be in order according to that column, allowing MySQL to simply retrieve the first 50 results from that index. No additional sorting required.

Disclaimer: I’m not a DBA. I may be completely wrong.

See Also: Mysql.com: Multiple Column Indexes