Re: index-only scans

On Fri, Aug 12, 2011 at 5:39 PM, Kevin Grittner
<Kevin.Grittner@wicourts.gov> wrote:
> Robert Haas <robertmhaas@gmail.com> wrote:
>
>> That's one of the points I asked for feedback on in my original
>> email.  "How should the costing be done?"
>
> It seems pretty clear that there should be some cost adjustment.  If
> you can't get good numbers somehow on what fraction of the heap
> accesses will be needed, I would suggest using a magic number based
> on the assumption that half the heap access otherwise necessary will
> be done.  It wouldn't be the worst magic number in the planner.  Of
> course, real numbers are always better if you can get them.

It wouldn't be that difficult (I think) to make VACUUM and/or ANALYZE
gather some statistics; what I'm worried about is that we'd have
correlation problems.  Consider a wide table with an index on (id,
name), and the query:

SELECT name FROM tab WHERE id = 12345

Now, suppose that we know that 50% of the heap pages have their
visibility map bits set.  What's the chance that this query won't need
a heap fetch?  Well, the chance is 50% *if* you assume that a row
which has been quiescent for a long time is just as likely to be
queried as one that has been recently inserted or updated.  However,
in many real-world use cases, nothing could be farther from the truth.

What do we do about that?

-- 
Robert Haas
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