On Tue, 25 May 2021 at 22:28, Andy Fan <zhihui.fan1213@gmail.com> wrote:
>
> explain (costs off) select (select count(*) filter (where t2.b = 1) from m1 t1)
> from m1 t2 where t2.b % 2 = 1;
>
> QUERY PLAN
> -------------------------------
> Aggregate
> -> Seq Scan on m1 t2
> Filter: ((b % 2) = 1)
> SubPlan 1
> -> Seq Scan on m1 t1
> (5 rows)
>
> This one is too confusing to me since the Aggregate happens
> on t2 rather than t1. What happens here? Would this query
> generate 1 row all the time like SELECT aggfunc(a) FROM t?
I think you're misreading the plan. There's a scan on t2 with a
subplan then an aggregate on top of that. Because you made the
subquery correlated by adding t2.b, it cannot be executed as an
initplan.
You might see what's going on better if you add VERBOSE to the EXPLAIN options.
Thanks, VERBOSE does provide more information.
Aggregate
Output: (SubPlan 1)
-> Seq Scan on public.m1 t2
Output: t2.a, t2.b
Filter: ((t2.b % 2) = 1)
SubPlan 1
-> Seq Scan on public.m1 t1
Output: count(*) FILTER (WHERE (t2.b = 1))
(8 rows)
I am still confused about the SubPlan1, how can it output a
count(*) without an Aggregate under it (If this is not easy to
explain, I can try more by myself later).
But after all, I find this case when working on the UniqueKey stuff,
I have rule that if (query->hasAgg && !query->groupClause), then
there are only 1 row for this query. In the above case, the outer query
(t2) hasAgg=true and subplan's hasAgg=false, which looks not right
to me. I think the hasAgg=true should be in the subquery and outer
query should have hasAgg=false. anything I missed?
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