More Efficient Way To Write MySQL Query? - mysql

Question

My site has suddenly started spitting out the following error:
"Incorrect key file for table '/tmp/#sql_645a_1.MYI'; try to repair it"
I remove it, the site works fine.
My server tech support guys suggest I clean up the query and make it more efficient.
Here's the query:
SELECT *, FROM_UNIXTIME(post_time, '%Y-%c-%d %H:%i') as posttime
FROM communityposts, communitytopics, communityusers
WHERE communityposts.poster_id=communityusers.user_id
AND communityposts.topic_id=communitytopics.topic_id
ORDER BY post_time DESC LIMIT 5
Any help is greatly appreciated. Perhaps can be done with a JOIN?
Many thanks,
Scott
UPDATE: Here's the working query, I still feel it could be optimised though.
SELECT
communityposts.post_id, communityposts.topic_id, communityposts.post_time,
communityusers.user_id, , communitytopics.topic_title, communityusers.username,
communityusers.user_avatar,
FROM_UNIXTIME(post_time, '%Y-%c-%d %H:%i') as post time
FROM
communityposts,
communitytopics,
communityusers
WHERE
communityposts.poster_id=communityusers.user_id
AND communityposts.topic_id=communitytopics.topic_id
ORDER BY post_time DESC LIMIT 5

Answer 1

SELECT
A.*,B.*,C.*,FROM_UNIXTIME(post_time, '%Y-%c-%d %H:%i') as posttime
FROM
(
SELECT id,poster_id,topic_id
FROM communityposts
ORDER BY post_time DESC
LIMIT 5
) cpk
INNER JOIN communityposts A USING (id)
INNER JOIN communityusers B ON cpk.poster_id=B.user_id
INNER JOIN communitytopics C USING (topic_id);
If a community post does not have to have a user and a topic, then use LEFT JOINs for the last two joins.
You will need to create a supporting index for the cpk subquqery:
ALTER TABLE communityposts ADD INDEX (posttime,id,poster_id,topic_id);
This query has to be the fastest because the cpk subquery only gets five keys ALL THE TIME.
UPDATE 2011-10-10 16:28 EDT
This query eliminiates the ambiguous topic_id issue:
SELECT
A.post_id, cpk.topic_id, A.post_time,
B.user_id, C.topic_title, B.username,
B.user_avatar,
FROM_UNIXTIME(post_time, '%Y-%c-%d %H:%i') as posttime
FROM
(
SELECT id,poster_id,topic_id
FROM communityposts
ORDER BY post_time DESC
LIMIT 5
) cpk
INNER JOIN communityposts A USING (id)
INNER JOIN communityusers B ON cpk.poster_id=B.user_id
INNER JOIN communitytopics C ON cpk.topic_id=C.topic_id;

Answer 2

Then temp table used for sorting the data probably gets too big. I have seen this happen when the /tmp/ runs out of space. The LIMIT clause does not make it any quicker or easier, as the sorting of the full data set has to be done first.
Under some conditions, MySQL does not use a temp table to sort data. You can read about it here: http://dev.mysql.com/doc/refman/5.0/en/order-by-optimization.html
If you manage to meet the right conditions (mostly use the correct indexes), it will also peed up your query.
If this doesn't help (in some cases you can't escape the heavy sorting), try to find out how much free space there is on /tmp/, and see if it can be expanded. Also, as sehe mentioned, selecting only the needed columns (instead of *) can make the temp table smaller and is considered best practice anyway (and so is using explicit JOINs instead of implicit ones).

Answer 3

You could reduce the number of fields selected.
The * operator will select all fields from all (3) tables. This may get big. That said,
I think mysql is smart enough to lay this plan out so that it doesn't need to access the data pages except for the 5 rows being selected.
Are you sure that all the involved (foreign) keys are indexed?
Here's my stab:
SELECT posts.*, FROM_UNIXTIME(post_time, '%Y-%c-%d %H:%i') as posttime
FROM communityposts posts
INNER JOIN communitytopics topics ON posts.topic_id = topics.topic_id
INNER JOIN communityusers users ON posts.poster_id = users.user_id
ORDER BY post_time DESC LIMIT 5