Re: partial indexes and bitmap scans

Tom,

* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> Stephen Frost <sfrost(at)snowman(dot)net> writes:
> > We have already figured out that the user's predicate results in a
> > subset of the index's or we wouldn't be able to use that index though,
> > right? Do we really need to spend cycles re-discovering that? Are
> > there cases where we actually need the index's predicate to ever be
> > included for correctness..?
>
> In a bitmap indexscan, yes, because the entire point of the recheck
> condition is that we're going to scan a whole page and possibly see
> tuples that don't satisfy the index predicate at all.

I understand the point of the recheck condition being that we might see
tuples that don't satisfy either the index predicate or the user's
predicate, but that isn't the point- to even consider using that index
we must have already realized that the user's predicate will only be
satisfied in a subset of cases when the index predicate predicate will
be satisfied, making any check of the index predicate necessairly always
true.

> Another point
> that's mentioned in the comments in createplan.c is that if you're
> considering the result of a BitmapOr or BitmapAnd, there's not necessarily
> a single index involved so it's much harder to reason about which part
> of the qual is an index predicate.

We must be pulling the index's predicate to be able to put it into the
Recheck condition in the first place. What I'm arguing is that once
we've decided that we're able to use an index X, because the values
which the user's predicate will satisfy is a subset of those which
the index's predicate will satisfy, then we can entirely forget about
the index's predicate as being redundant to the user's.

I don't see anything about a BitmapAnd or BitmapOr being relevant to
that. We will always need to check the user's predicate against the
tuples being returned from the Bitmap Heap Scan, unless by chance it
matches the index's predicate exactly *and* we have an entirely exact
match bitmap without any lossy parts.

> I do notice that createplan.c makes some effort to get rid of filter
> conditions that are provably true when the index conditions are.
> Doesn't look like it considers the reverse direction. Not sure if
> that's missing a bet.

That actually strikes me as a less likely condition to have in the
general case, isn't it? Wouldn't that only happen if the index
predicate and the user predicate are identical, because otherwise we
either can't use the index or we must keep the user's predicate because
it will only be satisfied in a subset of cases?

Thanks!

Stephen