From: | "A(dot) Kretschmer" <andreas(dot)kretschmer(at)schollglas(dot)com> |
---|---|
To: | Mark Mielke <mark(at)mark(dot)mielke(dot)cc>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: count * performance issue |
Date: | 2008-03-06 06:36:44 |
Message-ID: | 20080306063643.GB21084@a-kretschmer.de |
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Lists: | pgsql-performance |
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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