Re: Optimize query for listing un-read messages

På søndag 04. mai 2014 kl. 14:06:35, skrev Brice André <brice@famille-andre.be>:

Dear Andreas,
 

For me, putting both "LEFT OUTER JOIN" and "NOT EXISTS" is a bad idea.
 

As the "LEFT OUTER JOIN" will put fields of non-existing right table to null, I would simply rewrite it :
SELECT ... FROM message m
    LEFT OUTER JOIN message_property prop ON prop.message_id = m.id AND prop.person_id = 1
WHERE prop.is_read = TRUE
 

I would also ensure that an efficient index is used for the outer join. I would probably try at least a multi-column index on (message_id, person_id) for the property table. I would also maybe give a try to an index on (message_id, person_id, is_read), just to see if it improves performances.

 

The problem is that your suggested query doesn't return the desired results as it effectively is an INNER JOIN because you have "WHERE prop.is_read=TRUE", defeating the whole purpose of a LEFT OUTER JOIN.

 

--

Andreas Jospeh Krogh

CTO / Partner - Visena AS

Mobile: +47 909 56 963

andreas@visena.com

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