SELECT id, sender
FROM msgTable
WHERE sender=’John’
OR id IN (SELECT id FROM receiverTable WHERE receiver=’John’)

to the query with JOIN:

SELECT M.id, M.sender
FROM msgTable M LEFT JOIN receiverTable R ON M.id=R.id
WHERE M.sender=’John’
OR R.receiver=’John’

The OR’ed conditions worry me. Will the database smart enough to use the conditions separately for each table to limit the join size, or will it end up creating a large join table before applying the OR’ed conditions in WHERE? If it is the latter, will it better to just use nested queries, rather than JOIN?

Nested queries are still considered a poor practice in general, even if MySQL did close this issue. For general performance reasons, you should still avoid them in all DBMS’s whenever possible.

http://bugs.mysql.com/bug.php?id=17952

Better late than ever. Can you please update your article as well?

“SELECT *
FROM (
SELECT *
FROM messages
ORDER BY message_id DESC
LIMIT 15
)
ORDER BY message_id ASC”

Is it possible to write this as a join? I tried for hours but had to resort to this!

time difference for me: -0.005458 seconds compared to -0.545683 … That’s a 100 times faster!

Before:
SELECT * FROM `content`
WHERE `content`.`predicate_id` = ‘3’
AND `content`.`object_id` IN (‘x’,’y’,’z’)
AND `content`.`subject_id` IN
(SELECT `subject_id` FROM `content`
WHERE `content`.`predicate_id` = ‘some_var’
AND `content`.`object_id` = ‘:some_value’)

time: -0.545683

With join:
SELECT * FROM `content`
LEFT JOIN `content` AS `content2` on `content`.`subject_id` = `content2`.`subject_id`
WHERE `content`.`predicate_id` = ‘3’
AND `content`.`object_id` IN (‘x’,’y’,’z’)
AND `content2`.`predicate_id` = ‘some_var’
AND `content2`.`object_id` = ‘:some_value’

time: -0.005458!