From: Scott Marlowe <scott.marlowe@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: jay <jackem.mojx@alibaba-inc.com>
Cc: Heikki Linnakangas <heikki@enterprisedb.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] 答复: [PERFORM] Postgresql update op is very very slow
Date: Thu, 26 Jun 2008 10:02:31 -0600
Message-ID: <dcc563d10806260902o581ec3afud773867c1d0d2c69@mail.gmail.com> (raw)
In-Reply-To: <16213.1214492661@sss.pgh.pa.us>
References: <473587.97248.qm@web32701.mail.mud.yahoo.com>
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<004b01c8d671$3dec1630$0544000a@hz.ali.com>
<486219B6.7050003@enterprisedb.com>
<005101c8d773$ff81b2c0$0644000a@hz.ali.com>
<16213.1214492661@sss.pgh.pa.us>
2008/6/26 Tom Lane <tgl@sss.pgh.pa.us>:
> "jay" <jackem.mojx@alibaba-inc.com> writes:
>> I know the problem, because there are about 35 million rows , which
>> cost about 12G disk space and checkpoint segments use 64, but update
>> operation is in one transaction which lead fast fill up the checkpoint
>> segments and lead do checkpoints frequently, but checkpoints will cost lots
>> resources, so update operation become slowly and slowly and bgwrite won't
>> write because it's not commit yet.
>> Create a new table maybe a quick solution, but it's not appropriated in some
>> cases.
>> If we can do commit very 1000 row per round, it may resolve the
>> problem.
>
> No, that's utterly unrelated. Transaction boundaries have nothing to do
> with checkpoints.
True. But if you update 10000 rows and vacuum you can keep the bloat
to something reasonable.
On another note, I haven't seen anyone suggest adding the appropriate
where clause to keep from updating rows that already match. Cheap
compared to updating the whole table even if a large chunk aren't a
match. i.e.
... set col=0 where col <>0;
That should be the first thing you reach for in this situation, if it can help.
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To: pgsql-performance@postgresql.org
Cc: scott.marlowe@gmail.com, tgl@sss.pgh.pa.us, jackem.mojx@alibaba-inc.com, heikki@enterprisedb.com
Subject: Re: [PERFORM] 答复: [PERFORM] Postgresql update op is very very slow
In-Reply-To: <dcc563d10806260902o581ec3afud773867c1d0d2c69@mail.gmail.com>
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