Re: Optimize query for listing un-read messages - Mailing list pgsql-sql
På søndag 04. mai 2014 kl. 19:43:11, skrev Brice André <brice@famille-andre.be>:
The disadvantages of this solution are that the insertion of a new message or of a new message will be slower, and that your database size will be greater, but it should solve the problem of fast determining all read or unread messages of a dedicated user.To do so, you can use trigger mechanism on both the insertion of the message to create all message_property entries of that message, and on user insertion to create all message_properties of the user, so that you do not need to change anything outside your SQL design.If not, what I can propose your is to arrange yourself so that, for each couple (message, user) of your database, you have a corresponding entry in message_property, so that the first solution I proposed you (with an inner join) will work. And with multi-column indexes, it should be fast.I tried to figure out how you can avoid this with your current database design, but I did not found any solution. Maybe someone on the forum will have an idea.Forget my last answer : it was a stupid one... I tried to answer quickly, but with tiredness, it does not give good results.For me, your problem of performance comes from the "WHERE NOT EXISTS (query)" because your query is executed on each result of the outer join.
Yes, the reason it cannot be fast is because PG is unable to index the difference between two sets, so my schema, although a correct one, isn't index friendly so a caching-mechanism must be used for fast, indexed access. The solution is to redesign and have an entry in message_property for each combination of user/message.
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Andreas Jospeh Krogh
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