From: Tom Lane <tgl@sss.pgh.pa.us>
To: cluster <skrald@amossen.dk>
Cc: pgsql-performance@postgresql.org
Subject: Re: Two fast queries get slow when combined
Date: Wed, 31 Oct 2007 00:05:42 -0400
Message-ID: <8481.1193803542@sss.pgh.pa.us> (raw)
In-Reply-To: <fg85ae$1740$1@news.hub.org>
References: <fg85ae$1740$1@news.hub.org>
cluster <skrald@amossen.dk> writes:
> Queries and output from EXPLAIN ANALYZE can be seen here with some
> syntax highlighting:
> http://rafb.net/p/BJIW4p69.html
You are lying to us about how those queries were posed to Postgres
(and no I don't feel a need to explain how I know). In future please
present the full truth about what you are doing, not a simplification
that you think is sufficient.
But I think the short answer to your question is that query 1 is fast
because it need only select the first 50 rows in some ordering, and
query 2 is fast because it need only select the first 50 rows in
some ordering, but they are not the same ordering so the join query
doesn't get to exploit that shortcut.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, skrald@amossen.dk
Subject: Re: Two fast queries get slow when combined
In-Reply-To: <8481.1193803542@sss.pgh.pa.us>
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