Re: SeqScan costs

On Tue, 2008-08-12 at 23:58 +0100, Gregory Stark wrote:
> People lower random_page_cost because we're not doing a good job
> estimating how much of a table is in cache.

Is it because of a bad estimate about how much of a table is in cache,
or a bad assumption about the distribution of access to a table?

If the planner were to know, for example, that 10-20% of the table is
likely to be in cache, will that really make a difference in the plan? I
suspect that it would mostly only matter when the entire table is
cached, the correlation is low, and the query is somewhat selective
(which is a possible use case, but fairly narrow).

I suspect that this has more to do with the fact that some data is
naturally going to be accessed much more frequently than other data in a
large table. But how do you determine, at plan time, whether the query
will be mostly accessing hot data, or cold data?

Regards,
Jeff Davis