Re: Sequential scans

On Wed, 2007-05-02 at 23:59 +0100, Heikki Linnakangas wrote:

> Umm, you naturally have just entry per relation, but we were talking
> about how many entries the table needs to hold.. You're patch had a
> hard-coded value of 1000 which is quite arbitrary.

We need to think of the interaction with partitioning here. People will
ask whether we would recommend that individual partitions of a large
table should be larger/smaller than a particular size, to allow these
optimizations to kick in.

My thinking is that database designers would attempt to set partition
size larger than the sync scan limit, whatever it is. That means:
- they wouldn't want the limit to vary when cache increases, so we *do*
need a GUC to control the limit. My suggestion now would be
large_scan_threshold, since it effects both caching and synch scans.
- so there will be lots of partitions, so a hardcoded limit of 1000
would not be sufficient. A new GUC, or a link to an existing one, is
probably required.

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com