Re: Optimize query for listing un-read messages

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От	Pavel Stehule
Тема	Re: Optimize query for listing un-read messages
Дата	2 мая 2014 г. 00:03:08
Msg-id	CAFj8pRBiNroy_+Pa-7GnLR4vkoQNi6PHu1zva+=i4H-A0H4agQ@mail.gmail.com обсуждение исходный текст
Ответ на	Re: Optimize query for listing un-read messages (Andreas Joseph Krogh <andreas@visena.com>)
Ответы	Re: Optimize query for listing un-read messages
Список	pgsql-performance

Дерево обсуждения

2014-05-01 22:30 GMT+02:00 Andreas Joseph Krogh <andreas@visena.com>:

På torsdag 01. mai 2014 kl. 21:53:32, skrev Pavel Stehule <pavel.stehule@gmail.com>:

2014-05-01 21:39 GMT+02:00 Andreas Joseph Krogh <andreas@visena.com>:
På torsdag 01. mai 2014 kl. 21:30:39, skrev Pavel Stehule <pavel.stehule@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

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.

Regards

Pavel