Re: BUG #17618: unnecessary filter column <> text even after adding index

On Wed, Sep 21, 2022 at 7:56 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Flipping the removal around has the effect I expected on the plan shape,
> but some of the regression test queries now give the wrong answer, so
> there's something faulty about that analysis.

I think we may have a minor mistake when constructing the qpqual list in
create_bitmap_scan_plan. The qpqual list is supposed to be scan_clauses
minus indexquals. So we check each scan clause to see if it is redundant
with any indexqual, by using equal, checking EC or using
predicate_implied_by.

Note that the indexqual here may not be the form that has been going
through constant folding. Such as in this case with a boolean index, the
indexqual would be converted to 'indexkey expression = TRUE' by
match_boolean_index_clause. And that may make us fail to tell the scan
clause is redundant.

The comment of predicate_implied_by() says

* The top-level List structure of each list corresponds to an AND list.
* We assume that eval_const_expressions() has been applied and so there
* are no un-flattened ANDs or ORs (e.g., no AND immediately within an AND,
* including AND just below the top-level List structure).

So I think we need to run eval_const_expressions on indexquals before we
check for duplicate clauses, something like attached.

Thanks
Richard