Re: BUG #18825: Row value equality predicates do not use indices

On Tue, Feb 25, 2025 at 07:44:42PM -0500, Tom Lane wrote:
> PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> > DELETE FROM foo USING (SELECT dels FROM dels) AS dels
> > WHERE foo = dels; -- <--- does not use indices on either table
>
> [ shrug... ] Works for me given the full-row indexes, although I do
> have to force enable_seqscan off or reduce random_page_cost a lot,
> because otherwise the planner thinks a seqscan-and-sort is cheaper.

That seems like a bug, but a separate bug.

> I strongly suspect it's right, because full-row indexes are going to
> be bigger than the table proper. The mere fact that a plan uses
> indexes does not automatically make it better than one that doesn't.

If a full row index is a b-tree (or otherwise a prefix) index then one
think that skip-scanning the b-tree would be much faster than scanning
the table.

These are tables with two or three columns, all small, and the indices
are all covering indices, therefore the indices are all "bigger than the
table proper", yet decomposing the row equality predicate causes indices
to be used.

(Again, in this example all columns are NOT NULL.)

When I change the query to decompose the row equality predicate into
column equality predicates then the indices are used, and the query is
fast.

For the reasons I give above I suspect that a full-row b-tree index
would be equally fast, if only I could get the planner to try it.

> Anyway, AFAICS your gripe has nothing to do with "can the planner
> use these indexes", and everything to do with its cost estimates
> about the value of doing it that way.

I suspect that the planner doesn't know to try to decompose row equality
predicates into column equality predicates to take advantage of indices
on those columns. That would be a missing feature, not a bug, though I
admit that it was surprising (PG is so good that this sort of thing is
surprising, what can I say).

As for not using the full row index, from the evidence I have the
planner is indeed making a suboptimal choice here. Surely the planner
can't always make the optimal choice, but I suspect row equality
predicates are rare, and that when b-tree indices on full row values
exist then using those should have a slightly higher weight than not
using them.

Nico
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