Re: Synchronized scans

Tom Lane wrote:
> Heikki Linnakangas <heikki@enterprisedb.com> writes:
> > For the record, this patch has a small negative impact on scans like
> > "SELECT * FROM foo LIMIT 1000". If such a scan is run repeatedly, in CVS
> > HEAD the first 1000 rows will stay in buffer cache, but with the patch
> > each scan will start from roughly where previous one stopped, requiring
> > more pages to be read from disk each time. I don't think it's something
> > to worry about in practice, but I thought I'd mention it.
>
> Urgh.  The answers change depending on (more or less) the phase of the
> moon?  I've got a serious problem with that.  You might look back to
> 1997 when GEQO very nearly got tossed out entirely because it destroyed
> reproducibility of query results.

What about the simple idea of just disabling the use of a sync scan when
the query has LIMIT and no ORDER BY, and start always at block 0 in that
case?

--
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support