Re: Adding skip scan (including MDAM style range skip scan) to nbtree

On Fri, May 9, 2025 at 8:58 AM Tomas Vondra <tomas@vondra.me> wrote:
> My conclusion from this is that 92fe23d93aa ends up doing a lot of
> malloc calls, and this is what makes causes the regression. Otherwise
> setting the MALLOC_TOP_PAD_ would not help like this. But I haven't
> looked at the code, and I wouldn't have guessed the query to have
> anything to do with skip scan ...

While it's just about plausible that added nbtree preprocessing
allocation overhead could account for this, I don't think it's
actually possible here, since, as you said, this particular query
simply isn't eligible to use a skip scan. It's impossible for any
single column index to do so.

The best guess I have is that the skip scan commit inadvertently added
planner cycles, even though this query isn't eligible to use skip scan
-- I thought that I'd avoided that, but perhaps I overlooked some
subtlety.

Can you try it again, with prepared statements? Alternatively, you
could selectively revert the changes that commit 92fe23d93aa made to
utils/adt/selfuncs.c, and then retest.

--
Peter Geoghegan