From: A. Kretschmer <andreas.kretschmer@schollglas.com>
To: Mark Mielke <mark@mark.mielke.cc>
To: pgsql-performance@postgresql.org
Subject: Re: count * performance issue
Date: Thu, 6 Mar 2008 07:36:44 +0100
Message-ID: <20080306063643.GB21084@a-kretschmer.de> (raw)
In-Reply-To: <47CF8EA6.8070503@mark.mielke.cc>
References: <f966c2ee0803052142l6a105678jfe2da92a0d9371@mail.gmail.com>
<f966c2ee0803052143k4570d771o6c300647d3b987d@mail.gmail.com>
<20080306060829.GA21084@a-kretschmer.de>
<bf54be870803052215y5bb56235k5ea05ada1d701022@mail.gmail.com>
<47CF8EA6.8070503@mark.mielke.cc>
am Thu, dem 06.03.2008, um 1:26:46 -0500 mailte Mark Mielke folgendes:
>
>
> There aren't a general solution. If you realy need the exact count of
> tuples than you can play with a TRIGGER and increase/decrease the
> tuple-count for this table in an extra table.
>
>
> Of course, this means accepting the cost of obtaining update locks on the count
> table.
>
> The original poster should understand that they can either get a fast estimated
> count, or they can get a slow accurate count (either slow in terms of select
> using count(*) or slow in terms of updates using triggers and locking).
>
> Other systems have their own issues. An index scan may be faster than a table
> scan for databases that can accurately determine counts using only the index,
No. The current index-implementation contains no information about the
row-visibility within the current transaction. You need to scan the
whole data-table to obtain if the current row are visible within the
current transaction.
> but it's still a relatively slow operation, and people don't normally need an
> accurate count for records in the range of 100,000+? :-)
right.
Andreas
--
Andreas Kretschmer
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To: pgsql-performance@postgresql.org
Cc: andreas.kretschmer@schollglas.com, mark@mark.mielke.cc
Subject: Re: count * performance issue
In-Reply-To: <20080306063643.GB21084@a-kretschmer.de>
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