Re: SeqScan costs

On Tue, 2008-08-12 at 15:46 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> > Proposal: Make the first block of a seq scan cost random_page_cost, then
> > after that every additional block costs seq_page_cost.
>
> This is only going to matter for a table of 1 block (or at least very
> few blocks), and for such a table it's highly likely that it's in RAM
> anyway. So I'm unconvinced that the proposed change represents a
> better model of reality.

The access cost should be the same for a 1 block table, whether its on
disk or in memory.

> Perhaps more to the point, you haven't provided any actual evidence
> that this is a better approach. I'm disinclined to tinker with the
> fundamental cost models on the basis of handwaving.

I've written a simple test suite

psql -f seq.sql -v numblocks=x -v pkval=y -v filler=z

to investigate various costs and elapsed times.

AFAICS the cost cross-over is much higher than the actual elapsed time
cross-over for both narrow and wide tables.

Thats why using SET enable_seqscan=off helps performance in many cases,
or why people reduce random_page_cost to force index selection.

--
Simon Riggs www.2ndQuadrant.com
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