Re: BUG #17088: FailedAssertion in prepagg.c

On Thu, Jul 8, 2021 at 1:44 PM Richard Guo <guofenglinux@gmail.com> wrote:

>
> On Thu, Jul 8, 2021 at 5:56 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
>> Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
>> > "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:
>> >  Tom> Hmm. Maybe it'd be better if the default behavior in
>> >  Tom> expression_tree_walker/mutator did not include recursing into the
>> >  Tom> args, then?
>>
>> > You'd think, but as I recall (I will re-check this to confirm) there
>> > were more places where we _did_ need to recurse (especially during parse
>> > analysis before we've matched up the sortgrouprefs), while most of the
>> > places where recursion needed to be explicitly avoided already needed
>> > special-case handling, so having the default the other way would likely
>> > have required a special-case almost everywhere.
>>
>> Fair enough.  This is the kind of design choice that can be worth
>> revisiting later; but if the conclusion is still the same, fine with me.
>>
>
> I think the culprit is that when replacing correlation uplevel vars with
> Params, we do not handle the SubLinks in the arguments of uplevel
> GroupingFunc. We expect build_subplan should take care of it. But in
> build_subplan, we ignore GroupingFunc incorrectly.
>
> diff --git a/src/backend/optimizer/plan/subselect.c
> b/src/backend/optimizer/plan/subselect.c
> index 0881a208ac..e4918f275e 100644
> --- a/src/backend/optimizer/plan/subselect.c
> +++ b/src/backend/optimizer/plan/subselect.c
> @@ -364,7 +364,8 @@ build_subplan(PlannerInfo *root, Plan *plan,
> PlannerInfo *subroot,
>                  * SS_replace_correlation_vars).  Do that now.
>                  */
>                 if (IsA(arg, PlaceHolderVar) ||
> -                       IsA(arg, Aggref))
> +                       IsA(arg, Aggref) ||
> +                       IsA(arg, GroupingFunc))
>                         arg = SS_process_sublinks(root, arg, false);
>
>

I think we also need to change SS_process_sublinks to avoid recursing
into the arguments of an outer GroupingFunc. And that leads to a fix as
attached.

Thanks
Richard