To: Svetlin Manavski <svetlin.manavski@gmail.com>
cc: pgsql-performance@postgresql.org
Subject: Re: need to repeat the same condition on joined tables in order to
	choose the proper plan
In-reply-to: <BANLkTikSqgoucZkO0F3v8pHCopXjR=+NYg@mail.gmail.com> 
References: <BANLkTikSqgoucZkO0F3v8pHCopXjR=+NYg@mail.gmail.com>
Comments: In-reply-to Svetlin Manavski <svetlin.manavski@gmail.com>
	message dated "Tue, 14 Jun 2011 14:55:26 +0100"
Date: Tue, 14 Jun 2011 12:29:05 -0400
Message-ID: <676.1308068945@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Svetlin Manavski <svetlin.manavski@gmail.com> writes:
> I am really surprised to see that the planner needs me to explicitly specify
> the same condition twice like this:

>     SD.detectorid = SS.detectorid
>     and SD.sessionid = SS.id
>     and SD.detectorid = 1
>     and SD.sessionid >= 1001000000000::INT8 and SD.sessionid <=
> 2001000000000::INT8
>     and SS.detectorid = 1
>     and SS.id >= 1001000000000::INT8 and SS.id <= 2001000000000::INT8

The planner does infer implied equalities, eg, given A = B and B = C
it will figure out that A = C.  What you are asking is for it to derive
inequalities, eg infer A < C from A = B and B < C.  That would be
considerably more work for considerably less reward, since the sort of
situation where this is helpful doesn't come up very often.  On balance
I don't believe it's a good thing for us to do: I think it would make
PG slower on average because on most queries it would just waste time
looking for this sort of situation.

(In this example, the SS.detectorid = 1 clause is in fact unnecessary,
since the planner will infer it from SD.detectorid = SS.detectorid and
SD.detectorid = 1.  But it won't infer the range conditions on SS.id
from the range conditions on SD.sessionid or vice versa.)

			regards, tom lane