Re: Optimize query for listing un-read messages

På torsdag 01. mai 2014 kl. 23:02:13, skrev Pavel Stehule <
pavel(dot)stehule(at)gmail(dot)com <mailto:pavel(dot)stehule(at)gmail(dot)com>>:     2014-05-01 22:30
GMT+02:00 Andreas Joseph Krogh<andreas(at)visena(dot)com <mailto:andreas(at)visena(dot)com>>:
På torsdag 01. mai 2014 kl. 21:53:32, skrev Pavel Stehule <
pavel(dot)stehule(at)gmail(dot)com <mailto:pavel(dot)stehule(at)gmail(dot)com>>:     2014-05-01 21:39
GMT+02:00 Andreas Joseph Krogh<andreas(at)visena(dot)com <mailto:andreas(at)visena(dot)com>>:
På torsdag 01. mai 2014 kl. 21:30:39, skrev Pavel Stehule <
pavel(dot)stehule(at)gmail(dot)com <mailto:pavel(dot)stehule(at)gmail(dot)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
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