På lørdag 03. mai 2014 kl. 23:21:21, skrev Alban Hertroys <haram...@gmail.com <mailto:haram...@gmail.com>>: On 03 May 2014, at 12:45, Andreas Joseph Krogh <andr...@visena.com> wrote:
> Do you really need to query message_property twice? I would think this would give the same results: > > SELECT > m.id AS message_id, > prop.person_id, > coalesce(prop.is_read, FALSE) AS is_read, > m.subject > FROM message m > LEFT OUTER JOIN message_property prop ON prop.message_id = m.id AND prop.person_id = 1 AND prop.is_read = FALSE > ; Ah yes, of course that would match a bit too much. This however does give the same results: SELECT m.id AS message_id, prop.person_id, coalesce(prop.is_read, FALSE) AS is_read, m.subject FROM message m LEFT OUTER JOIN message_property prop ON prop.message_id = m.id AND prop.person_id = 1 WHERE prop.is_read IS NULL OR prop.is_read = FALSE ; That shaves off half the time of the query here, namely one indexscan. The remaining time appears to be spent finding the rows in “message" that do not have a corresponding “message_property" for the given (message_id, person_id) tuple. It’s basically trying to find no needle in a haystack, you won’t know that there is no needle until you’ve searched the entire haystack. It does seem to help a bit to create separate indexes on message_property.message_id and message_property.person_id; that reduces the sizes of the indexes that the database needs to match and merge other in order to find the missing message_id’s. I think the consesus here is to create a caching-table, there's no way around it as PG is unable to index the difference between two sets. -- Andreas Jospeh Krogh CTO / Partner - Visena AS Mobile: +47 909 56 963 andr...@visena.com <mailto:andr...@visena.com> www.visena.com <https://www.visena.com> <https://www.visena.com>
