Re: [PG19] Eager aggregation runs a volatile join clause once per group

Hi,

On Sun, Oct 12, 2026 shihao zhong wrote:
> Eager aggregation can put a partial aggregate below a join whose
> clause has a volatile function. The function then runs once per
> group, not once per joined row.

I tested v1 against master with a differential that runs each query
with eager aggregation on and off and counts the volatile function's
evaluations (a sequence bump), requiring both the eval count and the
result to match; a coverage guard checks that EXPLAIN really shows a
partial aggregate.

Over 23 placements of the volatile -- join ON, cross-rel WHERE, LEFT
JOIN, 3-way joins (upper and lower), CASE, correlated subquery,
LATERAL, semijoin, HAVING, plus partitionwise, parallel, window
functions and an SRF in the target list -- master miscounts in every
shape where the clause sits above the partial aggregate, and v1 fixes
all of them. For the parallel ones I confirmed the plans really were
parallel.

It is not over-broad: where the volatile sits below the partial
aggregate (3-way lower join, a WHERE on the grouped rel only, LATERAL,
...) v1 keeps eager aggregation and the count stays right, so it
refuses eager aggregation only where it must. The positions I
suspected statically (LATERAL, correlated subquery, semijoin, HAVING)
were already correct on master.

SQLancer (NoREC + TLP) on the v1 build ran clean, though it rarely
generates a volatile join clause, so that is weak evidence here.

Looks good to me.

Regards,
Manu