"Kevin Grittner" <kevin.gritt...@wicourts.gov> writes:
> Simon Riggs <si...@2ndquadrant.com> wrote:
>> This topic has been discussed many times, yet I have never seen an
>> assessment that explains WHY we would want to do index-only scans.
> In databases with this feature, it's not too unusual for a query
> which uses just an index to run one or more orders of magnitude
> faster than a query which has to randomly access the heap for each
> index entry.  That seems like enough evidence of its possible value
> in PostgreSQL to proceed to the point where benchmarks become
> possible.  I'm assuming that, like all other features added as
> performance optimizations, it won't be committed until there are
> benchmarks showing the net benefit.
> As a thought experiment, picture the relative costs of scanning a
> portion of an index in index sequence, and being done, versus
> scanning a portion of an index in index sequence and jumping to a
> random heap access for each index entry as you go.

It's already the case that we'll flip over to a bitmap indexscan,
and thus get rid of most/all of the "random" page accesses, in
situations where this is likely to be a big win.  Pointing to the
performance difference in databases that don't do that is therefore
not too convincing.

I'm inclined to agree that index-only scans might be worth the amount
of work that's involved ... but I share Simon's desire to see some proof
before anything gets committed.

                        regards, tom lane

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