From: Tom Lane <tgl@sss.pgh.pa.us>
To: Anton Maksimenkov <engineer@hlebprom.ru>
Cc: pgsql-performance@postgresql.org
Subject: Re: DELETE, INSERT vs SELECT, UPDATE || INSERT
Date: Fri, 23 Dec 2005 10:30:00 -0500
Message-ID: <19921.1135351800@sss.pgh.pa.us> (raw)
In-Reply-To: <287988431.20051223140205@hlebprom.ru>
References: <287988431.20051223140205@hlebprom.ru>
Anton Maksimenkov <engineer@hlebprom.ru> writes:
> Second idea was to try to SELECT (user, cookie) pair and then UPDATE
> it if it exists or INSERT if not. I has thought that if UPDATE will
> rewrite same place in file with new count it may lead to more compact
> table (file not grow and information about actual rows in file will
> not changed).
You're wasting your time, because Postgres doesn't work that way.
UPDATE is really indistinguishable from DELETE+INSERT, and there will
always be a dead row afterwards, because under MVCC rules both versions
of the row have to be left in the table for some time after your
transaction commits. See
http://www.postgresql.org/docs/8.1/static/mvcc.html
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, engineer@hlebprom.ru
Subject: Re: DELETE, INSERT vs SELECT, UPDATE || INSERT
In-Reply-To: <19921.1135351800@sss.pgh.pa.us>
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