From: Tom Lane <tgl@sss.pgh.pa.us>
To: Szűcs Gábor <surrano@gmail.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: fake condition causes far better plan
Date: Tue, 23 Aug 2005 11:14:19 -0400
Message-ID: <2638.1124810059@sss.pgh.pa.us> (raw)
In-Reply-To: <430B379A.7090204@gmail.com>
References: <430B379A.7090204@gmail.com>
=?ISO-8859-2?Q?Sz=FBcs_G=E1bor?= <surrano@gmail.com> writes:
> [ bad query plan ]
Most of the problem is here:
> -> Index Scan using muvelet_vonalkod_ny_idopont on
> muvelet_vonalkod_ny ny (cost=0.00..1351.88 rows=24649 width=4) (actual
> time=0.161..10.735 rows=3943 loops=1)
> Index Cond: (idopont >= (now() -
> ('00:00:00'::interval + ('1 days'::text)::interval)))
(BTW, you lied about the query, because this index condition doesn't
match anything in the given query text.)
Pre-8.0 releases aren't capable of making useful statistical estimates
for conditions involving nonconstant subexpressions, so you get a
badly-mistaken row count estimate that leads to a poor choice of plan.
If you can't update to 8.0, the best answer is to do the date arithmetic
on the client side. Another way is to create an allegedly-immutable
function along the lines of "ago(interval) returns timestamptz" to hide
the now() call --- this is dangerous but sometimes it's the easiest answer.
See the archives.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, surrano@gmail.com
Subject: Re: fake condition causes far better plan
In-Reply-To: <2638.1124810059@sss.pgh.pa.us>
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