Re: post-freeze damage control

Peter Geoghegan <pg(at)bowt(dot)ie> writes:
> On Mon, Apr 8, 2024 at 10:12 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I don't know that I'd call it scary exactly, but I do think it
>> was premature. A week ago there was no consensus that it was
>> ready to commit, but Alexander pushed it (or half of it, anyway)
>> despite that.

> Some of the most compelling cases for the transformation will involve
> path keys. If the transformation enables the optimizer to build a
> plain index scan (or index-only scan) with useful path keys, then that
> might well lead to a far superior plan compared to what's possible
> with BitmapOrs.

I did not say it isn't a useful thing to have. I said the patch
did not appear ready to go in.

> I understand that it'll still be possible to use OR expression
> evaluation in such cases, without applying the transformation (via
> filter quals), so in principle you don't need the transformation to
> get an index scan that can (say) terminate the scan early due to the
> presence of an "ORDER BY ... LIMIT n". But I suspect that that won't
> work out much of the time, because the planner will believe (rightly
> or wrongly) that the filter quals will incur too many heap page
> accesses.

That's probably related to the fact that we don't have a mechanism
for evaluating non-indexed quals against columns that are retrievable
from the index. We really oughta work on getting that done. But
I've been keeping a side eye on the work to unify plain and index-only
scans, because that's going to touch a lot of the same code so it
doesn't seem profitable to pursue those goals in parallel.

> Another problem (at least as I see it) with the new
> or_to_any_transform_limit GUC is that its design seems to have nothing
> to say about the importance of these sorts of cases. Most of these
> cases will only have 2 or 3 constants, just because that's what's most
> common in general.

Yeah, that's one of the reasons I'm dubious that the committed
patch was ready.

regards, tom lane