From: | secret <secret(at)kearneydev(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | PG-SQL <pgsql-sql(at)postgreSQL(dot)org>, John Ridout <johnridout(at)ctasystems(dot)co(dot)uk> |
Subject: | Re: [SQL] Good Optimization |
Date: | 1999-07-19 14:02:57 |
Message-ID: | 37933011.CB0F3E76@kearneydev.com |
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Lists: | pgsql-sql |
Tom Lane wrote:
> secret <secret(at)kearneydev(dot)com> writes:
> > There is a simple way to optimize SQL queries involving joins to
> > PostgreSQL that I think should be handled by Postgre? If one is joining
> > a tables a,b on attribute "x" and if one has something like x=3 then it
> > helps A LOT to say: a.x=3 and b.x=3 in addition to saying a.x=b.x ...
> > The example below shoulds the radical speed gain of doing this, and I
> > think it isn't something real obvious to most people...
>
> How much *actual* speedup is there? I don't trust the optimizer's
> numbers as anything more than relative measures ;-)
>
> I'm a bit surprised that you are getting a nested-loop plan and not
> a merge or hash join. With a merge join, at least, there ought not be
> a large difference from providing the additional qual clause (I think).
> What Postgres version are you using?
>
> regards, tom lane
The actual performance difference is HUGE. Hours vs minutes or Minutes vs
Seconds...
David Secret
MIS Director
Kearney Development Co., Inc.
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