Re: BUG #18652: Planner can not find pathkey item to sort for query with expression and expression index

On Thu, Oct 10, 2024 at 12:34 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> So it's specific to MergeAppend and it's been wrong from day zero.
> That makes me think it's probably not find_computable_ec_member's
> fault directly.  Fixing it there might be the most expedient answer,
> but I feel like first we should drill down a bit further to understand
> the root problem.  I'm too tired to do more tonight though.

Usually a relation's targetlist should include only Vars and PHVs
during this phase.  I think this may be the rationale behind
find_computable_ec_member matching only Vars and quasi-Vars.  However,
for a child rel, when we copy the parent's targetlist with appropriate
substitution, we may generate arbitrary expressions, such as an OpExpr
for 'i + 1' in this case.

We have a note in the comments in set_append_rel_size saying that

* Code that might be looking at an appendrel child must cope with
* such.

It seems to me that find_computable_ec_member does not get this memo.

Alternatively, can we wrap non-var expressions in a childrel's
targetlist into PHVs, so that we do not need special handling for an
appendrel child?  I have not tried this though.

Thanks
Richard