(2018/07/11 20:02), Ashutosh Bapat wrote:
> On Wed, Jul 11, 2018 at 1:23 PM, Etsuro Fujita
> <fujita.etsuro@lab.ntt.co.jp> wrote:
>> Actually, even if we could create such an index on the child table and the
>> targetlist had the ConvertRowtypeExpr, the planner would still not be able
>> to use an index-only scan with that index; because check_index_only would
>> not consider that an index-only scan is possible for that index because that
>> index is an expression index and that function currently does not consider
>> that index expressions are able to be returned back in an index-only scan.
>> That behavior of the planner might be improved in future, though.
> Right and when we do so, not having ConvertRowtypeExpr in the
> targetlist will be a problem.
Yeah, but I don't think that that's unsolvable; because in that case the
CRE as an index expression could be converted back to the whole-row
Var's rowtype by adding another CRE to the index expression for that
conversion, I suspect that that special handling could allow us to
support an index-only scan even when having the whole-row Var instead of
the CRE in the targetlist. (Having said that, I'm not 100% sure we need
to solve that problem when we improve the planner, because there doesn't
seem to me to be enough use-case to justify making the code complicated
for that.) Anyway, I think that that would be a matter of future
versions of PG.
>>> At places in planner we match equivalence members
>>> to the targetlist entries. This matching will fail unexpectedly when
>>> ConvertRowtypeExpr is removed from a child's targetlist. But again I
>>> couldn't reproduce a problem when such a mismatch arises.
>>
>>
>> IIUC, I don't think the planner assumes that for an equivalence member there
>> is an matching entry for that member in the targetlist; what I think the
>> planner assumes is: an equivalence member is able to be computed from
>> expressions in the targetlist.
>
> This is true. However,
>
>> So, I think it is safe to have whole-row
>> Vars instead of ConvertRowtypeExprs in the targetlist.
>
> when it's looking for an expression, it finds a whole-row expression
> so it think it needs to add a ConvertRowtypeExpr on that. But when the
> plan is created, there is ConvertRowtypeExpr already, but there is no
> way to know that a new ConvertRowtypeExpr is not needed anymore. So,
> we may have two ConvertRowtypeExprs giving wrong results.
Maybe I'm missing something, but I don't think that we need to worry
about that, because in the approach I proposed, we only add CREs above
whole-row Vars in the targetlists for subplans of an Append/MergeAppend
for a partitioned relation at plan creation time.
Best regards,
Etsuro Fujita
