Re: Tuple count used while costing MergeAppend and that for an append rel

>
> AFAICS, what you propose to add in set_append_rel_size is pure overhead.
> There's no conceivable use to computing sum-of-raw-tuple-counts for an
> appendrel ... or at least, if there is, you didn't say what you expect
> it would be good for. Normally the difference between rel->tuples and
> rel->rows corresponds to the selectivity of the rel's restriction clauses.
> Since an appendrel has no restrictions of its own (they've all been pushed
> down to the child rels) it doesn't seem unreasonable for it to have tuples
> equal to rows. While we could also define it as you suggest, I don't see
> the point of expending extra cycles to do so.
>

I suggested that change, just to keep the computation consistent with
the comment in RelOptInfo. I agree that it's a pure overhead for now.
If we were to maintain it per the comments, we may be able to find the
average selectivity for an appendrel, but again, I do not see any real
purpose in doing so.

> I concur that using path->rows not rel->tuples in create_merge_append_path
> would be an improvement. I think it makes no difference now, but if we
> were to support parameterized mergeappend paths, rel->rows and rel->tuples
> would both be wrong for such a path. (I believe the possibility of a
> parameterized path is the reason why create_append_path computes
> path->rows the hard way instead of just copying it from rel->rows.
> The logic in create_merge_append_path was probably just copied from
> create_append_path; it's overkill today but might not be so forever.)

Yes, this code did puzzle me. add_path() does not allow paths to have
both parameterization and pathkeys. So, a merge append on
parameterized paths does look odd. But then, I thought, that producing
a parameterized merge append would get allow merging ordered rows for
a given value of parameter. Although we don't do that now, but may be
it's useful in the future.

Thanks a lot for your explanation.
--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company