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

On 10.10.2024 14:57, Richard Guo wrote:
> 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.
>
To be honest, looking at the MergeAppend processing plan, it’s not 
entirely clear to me what the functional difference is between 
set_rel_pathlist and

find_computable_ec_member. Both prepare pathkeys , but what the first 
function doesn't do is what the other one needs to do?Could you explain 
it to me?


-- 
Regards,
Alena Rybakina
Postgres Professional