From: Jakub Ouhrabka <kuba@comgate.cz>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Subject: Re: Linux/PostgreSQL scalability issue - problem with 8 cores
Date: Tue, 15 Jan 2008 18:06:33 +0100
Message-ID: <478CE819.4060409@comgate.cz> (raw)
In-Reply-To: <24676.1200414360@sss.pgh.pa.us>
References: <flii57$748$1@news.hub.org>
<7083.1199379570@sss.pgh.pa.us>
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<26502.1199745090@sss.pgh.pa.us>
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<27655.1199750492@sss.pgh.pa.us>
<47836A4C.5080707@comgate.cz>
<10713.1199813111@sss.pgh.pa.us>
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<24676.1200414360@sss.pgh.pa.us>
> Okay, for a table of just a few entries I agree that DELETE is
> probably better. But don't forget you're going to need to have those
> tables vacuumed fairly regularly now, else they'll start to bloat.
I think we'll go with DELETE also for another reason:
Just after we figured out the cause of the spikes we started to
investigate a long-term issue we had with PostgreSQL: pg_dump of big
database was blocking some of our applications. And yes, we replaced
TRUNCATE with DELETE and everything is running as expected.
Looking at the docs now I see there is a new paragraph in 8.3 docs
mentioning that TRUNCATE is not MVCC-safe and also the blocking issue.
It's a pity that the warning wasn't there in 7.1 times :-)
Thanks,
Kuba
Tom Lane napsal(a):
> Jakub Ouhrabka <kuba@comgate.cz> writes:
>>>> Huh. One transaction truncating a dozen tables? That would match the
>>>> sinval trace all right ...
>
>> It should be 4 tables - the shown log looks like there were more truncates?
>
> Actually, counting up the entries, there are close to 2 dozen relations
> apparently being truncated in the trace you showed. But that might be
> only four tables at the user level, since each index on these tables
> would appear separately, and you might have a toast table plus index
> for each one too. If you want to dig down, the table OIDs are visible
> in the trace, in the messages with type -1:
>
>>> LOG: sending inval msg -1 0 30036 0 30700 3218341912
> ^^^^^ ^^^^^
> DBOID RELOID
>
> so you could look into pg_class to confirm what's what.
>
>> Yes, performance was the initial reason to use truncate instead of
>> delete many years ago. But today the truncated tables usualy contain
>> exactly one row - quick measurements now show that it's faster to issue
>> delete instead of truncate in this case.
>
> Okay, for a table of just a few entries I agree that DELETE is probably
> better. But don't forget you're going to need to have those tables
> vacuumed fairly regularly now, else they'll start to bloat.
>
> regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: kuba@comgate.cz, tgl@sss.pgh.pa.us
Subject: Re: Linux/PostgreSQL scalability issue - problem with 8 cores
In-Reply-To: <478CE819.4060409@comgate.cz>
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