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Re: CLUSTER and synchronized scans and pg_dump et al

From: Jeff Davis <pgsql(at)j-davis(dot)com>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: pgsql-hackers list <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: CLUSTER and synchronized scans and pg_dump et al
Date: 2008-01-28 22:38:50
Message-ID: 1201559930.10057.641.camel@dogma.ljc.laika.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sun, 2008-01-27 at 15:02 +0000, Gregory Stark wrote:
> It occurred to me the other day that synchronized scans could play havoc with
> clustered tables. When you dump and reload a table even if it was recently
> clustered if any other sequential scans are happening in the system at the
> time you dump it the dump could shuffle the records out of order. 
> 
> Now the records would still be effectively ordered for most purposes but our
> statistics can't detect that. Since the correlation would be poor the restored
> database would have markedly different statistics showing virtually no
> correlation on the clustered column.
> 
> Perhaps we should have some form of escape hatch for pg_dump to request real
> physical order when dumping clustered tables.
> 

Thank you for bringing this up, it's an interesting point.

Keep in mind that this only matters if you are actually running pg_dump
concurrently with another scan, because a scan will reset the starting
point after completing.

Regards,
	Jeff Davis


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