Re: SeqScan costs

"Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:

> 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.

I think the first block of a sequential scan is clearly a random access. If
that doesn't represent reality well then perhaps we need to tackle both
problems together. Somehow we need to discount scan i/o cost based on how much
of the table we expect to be in cache. For 1-block tables if we should expect
them to be in cache we should be zeroing out all the i/o cost whether random
or sequential.

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

Uhm, huh? That can't be what you meant to write?

> 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.

People lower random_page_cost because we're not doing a good job estimating
how much of a table is in cache. I think that would be a great target for some
careful analysis. If you can come up with specific places and reasonable
heuristics to discount i/o costs based on effective_cache_size and then
demonstrate cases where it produces consistently better cost estimates that
would be a huge help.

I've been running benchmarks where I see accurate random_page_costs of 13-80
on uncached data on a moderate sized raid array. But of course when a some of
the data is cached the effective random_page_cost is much much lower than
that.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
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