Re: I/O on select count(*)

Alvaro Herrera wrote:
> Greg Smith escribió:
>> On Thu, 15 May 2008, Pavan Deolasee wrote:
>>
>>> I had suggested in the past that whenever we set hint bits for a tuple,
>>> we should check all other tuples in the page and set their hint bits
>>> too to avoid multiple writes of the same page. I guess the idea got
>>> rejected because of lack of benchmarks to prove the benefit.
>>
>> From glancing at http://www.postgresql.org/docs/faqs.TODO.html I got the
>> impression the idea was to have the background writer get involved to
>> help with this particular situation.
>
> The problem is that the bgwriter does not understand about the content
> of the pages it is writing -- they're opaque pages for all it knows. So
> it cannot touch the hint bits.

We know what kind of a relation we're dealing with in ReadBuffer, so we
could add a flag to BufferDesc to mark heap pages.

> If we had the bitmask in a separate map fork, this could be cheap.

I don't buy that. The point of a hint bit is that it's right there along
with the tuple you're looking at. If you have to look at a separate
buffer, you might as well just look at clog.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com