From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Lucas Madar <madar@samsix.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Poor performance when joining against inherited tables
Date: Wed, 11 May 2011 13:00:26 -0400
Message-ID: <508.1305133226@sss.pgh.pa.us> (raw)
In-Reply-To: <BANLkTimssX0BS_ycZjye3z7TLiiiPqi8tg@mail.gmail.com>
References: <sig.50826e4e2f.4DA3608E.9090906@samsix.com>
<BANLkTimssX0BS_ycZjye3z7TLiiiPqi8tg@mail.gmail.com>
Robert Haas <robertmhaas@gmail.com> writes:
> A more interesting question is why you're not getting a plan like this:
> Nested Loop
> -> Seq Scan on objects
> -> Append
> -> Index Scan using xxx_pkey on itemXX
> -> Index Scan using yyy_pkey on itemYY
> -> Index Scan using zzz_pkey on itemZZ
Probably because there are 4 million rows in the objects table.
Or maybe it's a pre-8.2 database and can't even generate such a plan.
But if it did generate it, it would almost certainly have decided that
this was more expensive than a hash or merge join.
People have this weird idea that the existence of an index ought to make
enormous joins free ...
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, robertmhaas@gmail.com, madar@samsix.com
Subject: Re: Poor performance when joining against inherited tables
In-Reply-To: <508.1305133226@sss.pgh.pa.us>
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