Re: reducing random_page_cost from 4 to 2 to force index scan

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, May 16, 2011 at 12:49 AM, Jesper Krogh <jesper(at)krogh(dot)cc> wrote:
>> Ok, it may not work as well with index'es, since having 1% in cache may very
>> well mean that 90% of all requested blocks are there.. for tables in should
>> be more trivial.

> Tables can have hot spots, too. Consider a table that holds calendar
> reservations. Reservations can be inserted, updated, deleted. But
> typically, the most recent data will be what is most actively
> modified, and the older data will be relatively more (though not
> completely) static, and less frequently accessed. Such examples are
> common in many real-world applications.

Yes. I'm not convinced that measuring the fraction of a table or index
that's in cache is really going to help us much. Historical cache hit
rates might be useful, but only to the extent that the incoming query
has a similar access pattern to those in the (recent?) past. It's not
an easy problem.

I almost wonder if we should not try to measure this at all, but instead
let the DBA set a per-table or per-index number to use, analogous to the
override we added recently for column n-distinct statistics ...

regards, tom lane