På torsdag 01. mai 2014 kl. 23:02:13, skrev Pavel Stehule < pavel.steh...@gmail.com <mailto:pavel.steh...@gmail.com>>: 2014-05-01 22:30 GMT+02:00 Andreas Joseph Krogh<andr...@visena.com <mailto:andr...@visena.com>>: På torsdag 01. mai 2014 kl. 21:53:32, skrev Pavel Stehule < pavel.steh...@gmail.com <mailto:pavel.steh...@gmail.com>>: 2014-05-01 21:39 GMT+02:00 Andreas Joseph Krogh<andr...@visena.com <mailto:andr...@visena.com>>: På torsdag 01. mai 2014 kl. 21:30:39, skrev Pavel Stehule < pavel.steh...@gmail.com <mailto:pavel.steh...@gmail.com>>: Hello [snip] I had a perfect success on similar use case with descent ordered partial index
http://www.postgresql.org/docs/9.3/interactive/sql-createindex.html <http://www.postgresql.org/docs/9.3/interactive/sql-createindex.html> I'm not getting good performance. Are you able to craft an example using my schema and partial index? maybe some like CREATE INDEX ON message_property (person_id, message_id) WHERE pr.is_read When I am thinking about your schema, it is designed well, but it is not index friendly, so for some fast access you should to hold a cache (table) of unread messages Ah, that's what I was hoping to not having to do. In my system, messages arrive all the time and having to update a cache for all new messages for all users seems messy... Seems I could just as well create a message_property for all users when a new message arrives, so I can INNER JOIN it and get good performance. But that table will quickly grow *very* large... What you need is a JOIN index, that is not possible in Postgres. I afraid so some "ugly" solutions is necessary (when you require extra fast access). You need a index (small index) and it require some existing set - you cannot do index on the difference two sets. I expect so some flag on the relation "message" - like "it should not be not read" can helps little bit - and can be used in partial index as conditions. Other possibility is some variant of partitioning - you can divide a messages and users to distinct sets and then you decrease a number of possible combinations. Just curious: Is such a JOIN index possible in other DBs, if so - which? Can other DBs do index on difference between two sets? Will PG ever have this, is it even possible? In my real system the message_property holds other properties for a message also, so I have to LEFT OUTER JOIN with it to get the properties where they exist when listing messages. The system works by assuming that when an entry in message_property does not exist for a given message for a given user then the property is equal to "false". I don't quite see how maintaining a message_properrty entry for all users, for all messages, is a good idea. That's quite some work to be done when adding/removing users etc. Thanks for having this discussion. -- 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>
