Robert Haas <robertmh...@gmail.com> writes:
> That's a bit disappointing - it's now more than a third faster to do
> the sequential scan, even though the sequential scan has to touch six
> times as many blocks (at scale factor 20, index is 43 MB, table is 256
> MB) all of which are in cache. Of course, touching that many fewer
> blocks does have some advantages if there is concurrent activity on
> the system, but it still seems unfortunate that the ratio of runtime
> to blocks touched is more than 8x higher for the index-only case.
I don't know why you'd imagine that touching an index is free, or even
cheap, CPU-wise. The whole point of the index-only optimization is to
avoid I/O. When you try it on a case where there's no I/O to be saved,
*and* no shared-buffers contention to be avoided, there's no way it's
going to be a win.
regards, tom lane
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