选择耗时8秒。 改善想法

Question

I have this select to get chat (like facebook inbox). It will show most recent messages, grouping by user who sent them.

SELECT c.id, c.from, c.to, c.sent, c.message, c.recd FROM chat c 
WHERE c.id IN(
  SELECT MAX(id) FROM chat
  WHERE (`to` = 1 and `del_to_status` = '0') or (`from` = 1 and `del_from_status` = '0')
  GROUP BY CASE WHEN 1 = `to` THEN `from` ELSE `to` END
)
ORDER BY id DESC
limit 60

The problem is it is taking about 8 seconds.

`chat` (
  `id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
  `from` int(11) UNSIGNED NOT NULL,
  `to` int(11) UNSIGNED NOT NULL,
  `message` text NOT NULL,
  `sent` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `recd` tinyint(1) NOT NULL DEFAULT '0',
  `del_from_status` tinyint(1) NOT NULL DEFAULT '0',
  `del_to_status` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `from` (`from`),
  KEY `to` (`to`),
  FOREIGN KEY (`from`) REFERENCES cadastro (`id`),
  FOREIGN KEY (`to`) REFERENCES cadastro (`id`)
)

any ideas of indexing or re-writing this select to get better speed?

Answer 1

I am assuming chat.id is indexed. If not, of course you should add an index.

If it is indexed, MySQL is often very slow with sub selects.

One thing you can do is convert your sub select to a temporary table and join with it.

It will look something like

    CREATE TEMPORARY TABLE IF NOT EXISTS max_chat_ids
                ( INDEX(id) ) 
                ENGINE=MEMORY
                AS ( 'SELECT MAX(id) as id FROM chat
  WHERE (`to` = 1 and `del_to_status` = '0') or (`from` = 1 and `del_from_status` = '0')
  GROUP BY CASE WHEN 1 = `to` THEN `from` ELSE `to` END' );

then, you need to just join with the temp table:

SELECT c.id, c.from, c.to, c.sent, c.message, c.recd FROM chat c 
join max_chat_ids d on c.id=d.id
ORDER BY c.id DESC
limit 60

temp tables only live during the duration of the session, so if you test this in phpmyadmin remember to execute both queries together with ';' between them.

If you try this share your result.

Answer 2

I'll assume the column id is already indexed since it probably is the primary key of the table. If it's not the case, add the index:

create index ix1_chat on chat (id);

Then, if the selectivity of the subquery is good then an index will help. The selectivity is the percentage of rows the select is reading compared to the total number of rows. Is it 50%, 5%, 0.5%? If it's 5% or less then the following index will help:

create index ix2_chat on chat (`to`, del_to_status, `from`, del_from_status);

As a side note, please don't use reserved words for column names: I'm talking about the from column. It just makes life difficult for everyone.