Date: Tue, 8 May 2007 12:23:56 +0200 (CEST)
From: Pomarede Nicolas <npomarede@corp.free.fr>
To: Heikki Linnakangas <heikki@enterprisedb.com>
cc: pgsql-performance@postgresql.org
Subject: Re: truncate a table instead of vaccum full when count(*)
 is 0
In-Reply-To: <46404BEA.5060009@enterprisedb.com>
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	<46404BEA.5060009@enterprisedb.com>
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On Tue, 8 May 2007, Heikki Linnakangas wrote:

> Pomarede Nicolas wrote:
>> But for the data (dead rows), even running a vacuum analyze every day is 
>> not enough, and doesn't truncate some empty pages at the end, so the data 
>> size remains in the order of 200-300 MB, when only a few effective rows are 
>> there.
>
> For a table like that you should run VACUUM much more often than once a day. 
> Turn on autovacuum, or set up a cron script etc. to run it every 15 minutes 
> or so.

Yes, I already do this on another spool table ; I run a vacuum after 
processing it, but I wondered if there was another way to keep the disk 
size low for this table.

As for autovacuum, the threshold values to analyze/vacuum are not adapted 
to my situation, because I have some big tables that I prefer to keep 
vacuumed frequently to prevent growing in disk size, even if the number of 
insert/update is not big enough and in my case autovacuum would not run 
often enough. Instead of configuring autovacuum on a per table basis, I 
prefer running a vacuum on the database every day.


>
>> Running a vacuum full is a solution for now, but it locks the table for too 
>> long (10 minutes or so), which is not acceptable in that case, since events 
>> should be processed in less that 10 seconds.
>> 
>> So, I would like to truncate the table when the number of rows reaches 0 
>> (just after the table was processed, and just before some new rows are 
>> added).
>> 
>> Is there an easy way to do this under psql ? For example, lock the table, 
>> do a count(*), if result is 0 row then truncate the table, unlock the table 
>> (a kind of atomic 'truncate table if count(*) == 0').
>> 
>> Would this work and what would be the steps ?
>
> It should work, just like you describe it, with the caveat that TRUNCATE will 
> remove any old row versions that might still be visible to an older 
> transaction running in serializable mode. It sounds like it's not a problem 
> in your scenario, but it's hard to say for sure without seeing the 
> application. Running vacuum more often is probably a simpler and better 
> solution, anyway.
>
> Which version of PostgreSQL is this?

Shouldn't locking the table prevent this ? I mean, if I try to get an 
exclusive lock on the table, shouldn't I get one only when there's no 
older transaction, and in that case I can truncate the table safely, 
knowing that no one is accessing it due to the lock ?

the pg version is 8.1.2 (not the latest I know, but migrating this base is 
quite complicated since it needs to be up 24/24 a day)

thanks

Nicolas