Re: POC, WIP: OR-clause support for indexes

On Mon, Jun 24, 2024 at 1:29 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> I am not against handling this kind of case if we can do it, but it's
> more important that the patch doesn't cause gratuitous failures than
> that it handles more cases.

I agree, with the proviso that "avoid gratuitous failures" should
include cases where a query that got the optimization suddenly fails
to get the optimization, due only to some very innocuous looking
change. Such as a change from using a constant 1_000_000_000 to a
constant 5_000_000_000 in the query text. That is a POLA violation.

> > Maybe it would be practical to do something with the B-Tree operator
> > class for each of the types involved in the optimization. You could
> > probably find a way for a SAOP to work against a
> > "heterogeneously-typed array" while still getting B-Tree index scans
> > -- provided the types all came from the same operator family. I'm
> > assuming that the index has an index column whose input opclass was a
> > member of that same family. That would necessitate changing the
> > general definition of SAOP, and adding new code to nbtree that worked
> > with that. But that seems doable.
>
> I agree that something based on operator families might be viable. Why
> would that require changing the definition of an SAOP?

Maybe it doesn't. My point was only that the B-Tree code doesn't
necessarily need to use just one rhs type for the same column input
opclass. The definition of SOAP works (or could work) in basically the
same way, provided the "OR condition" were provably disjunct. We could
for example mix different operators for the same nbtree scan key (with
some work in nbtutils.c), just as we could support "where mycol =5 OR
mycol IS NULL" with much effort.

BTW, did you know MySQL has long supported the latter? It has a <=>
operator, which is basically a non-standard spelling of IS NOT
DISTINCT FROM. Importantly, it is indexable, whereas right now
Postgres doesn't support indexing IS NOT DISTINCT FROM. If you're
interested in working on this problem within the scope of this patch,
or some follow-up patch, I can take care of the nbtree side of things.

> > Admittedly I'm glossing over a lot of important details here. Does it
> > just work for the default opclass for the type, or can we expect it to
> > work with a non-default opclass when that's the salient opclass (the
> > one used by our index)? I don't know what you'd do about stuff like
> > that.
>
> It seems to me that it just depends on the opclasses in the query. If
> the user says
>
> WHERE column op1 const1 AND column op2 const2
>
> ...then if op1 and op2 are in the same operator family and if we can
> convert one of const1 and const2 to the type of the other without risk
> of failure, then we can rewrite this as an SAOP with whichever of the
> two operators pertains to the target type, e.g.
>
> column1 op1 ANY[const1,converted_const2]
>
> I don't think the default opclass matters here, or the index properties either.

Okay, good.

The docs do say "Another requirement for a multiple-data-type family
is that any implicit or binary-coercion casts that are defined between
data types included in the operator family must not change the
associated sort ordering" [1]. There must be precedent for this sort
of thing. Probably for merge joins.

[1] https://www.postgresql.org/docs/devel/btree.html#BTREE-BEHAVIOR
--
Peter Geoghegan