pub type GroupMap = (ColumnName, BTreeMap<String, Value<'static>>, Value<'static>);Expand description
v7.29 - one batch-evaluated correlated subquery: the outer key column and the key -> value map.
v7.37.x (docker-fair SCALARSQ attack) — extended with an
empty_default Value. PG scalar-subquery empty-set semantics
distinguish COUNT(*) / COUNT(col) (= 0 over no rows) from
every other aggregate (= NULL). The hollow_scalar_subqueries
template-rewrite step empties the inner SelectStatement before
the per-row splice, so the splicer cannot inspect the original
aggregate kind at probe time. Storing the empty-default on the
GroupMap captures that information at try_batch_correlated_scalar
construction time, where the original inner is still in hand.