Re: GIN indexes on an = ANY(array) clause

Corey Huinker <corey(dot)huinker(at)gmail(dot)com> writes:
> A client had an issue with a where that had a where clause something like
> this:
> WHERE 123456 = ANY(integer_array_column)
> I was surprised that this didn't use the pre-existing GIN index on
> integer_array_column, whereas recoding as
> WHERE ARRAY[123456] <@ integer_array_column
> did cause the GIN index to be used. Is this a known/expected behavior? If
> so, is there any logical reason why we couldn't have the planner pick up on
> that?
> Flo Rance (tourance(at)gmail(dot)com) was nice enough to show that yes, this is
> expected behavior.

The planner doesn't know enough about the semantics of array <@ to make
such a transformation. (As pointed out in the stackoverflow article Flo
pointed you to, the equivalence might not even hold, depending on which
version of <@ we're talking about.)

Since the GIN index type is heavily oriented towards array-related
operators, I spent some time wondering whether we could get any mileage
by making ScalarArrayOpExpr indexquals be natively supported by GIN
(right now they aren't). But really I don't see where the GIN AM would
get the knowledge from, either. What it knows about the array_ops
opclass is basically the list of associated operators:

regression=# select amopopr::regoperator from pg_amop where amopfamily = 2745;
amopopr
-----------------------
&&(anyarray,anyarray)
@>(anyarray,anyarray)
<@(anyarray,anyarray)
=(anyarray,anyarray)
(4 rows)

and none of those are obviously related to the =(int4,int4) operator that
is in the ScalarArrayOp. The only way to get from point A to point B is
to know very specifically that =(anyarray,anyarray) is related to any
scalar-type btree equality operator, which is not the kind of thing the
GIN AM ought to know either.

Really the array_ops opclass itself is the widest scope where it'd be
reasonable to embed knowledge about this sort of thing --- but we lack
any API at all whereby opclass-specific code could affect planner behavior
at this level. Even if we had one, there's no obvious reason why we
should be consulting a GIN opclass about a ScalarArrayOp that does not
contain an operator visibly related to the opclass. That path soon
leads to consulting everybody about everything and planner performance
going into the tank.

Extensibility is a harsh mistress.

regards, tom lane