Subject: pgsql: Fix EXPLAIN failure when deparsing SQL/JSON aggregates

From: Richard Guo <[email-masked]>
Date: Tue, 07 Jul 2026 23:51:50 +0000

Fix EXPLAIN failure when deparsing SQL/JSON aggregates

If an expression containing an aggregate is evaluated above the plan
node that computes the aggregate, as happens with window functions or
with expressions postponed to above the final sort, setrefs.c replaces
the Aggref or WindowFunc with a Var referencing the lower node's
output.  For SQL/JSON aggregates such as JSON_ARRAYAGG and
JSON_OBJECTAGG, deparsing the containing JsonConstructorExpr then
failed with "invalid JsonConstructorExpr underlying node type", since
get_json_agg_constructor() did not expect a Var there.

Fix by resolving the Var back to the underlying Aggref or WindowFunc
and deparsing the constructor as if the aggregate were computed at the
current node.  The JsonConstructorExpr retains the RETURNING clause
and the ABSENT/NULL ON NULL and WITH UNIQUE options, and the arguments
come from the resolved aggregate, so the original JSON aggregate
syntax is reproduced in full.  This mirrors how get_agg_expr() already
looks through such a Var when deparsing a combining aggregate.

Reported-by: Thom Brown <[email-masked]>
Author: Richard Guo <[email-masked]>
Discussion: https://postgr.es/m/CAA-aLv5QYTaMOk=[email-masked]
Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/485527190a97f7eb57a1ce28e140d9e5f3f60f0a

Modified Files
--------------
src/backend/utils/adt/ruleutils.c     | 32 ++++++++++++
src/test/regress/expected/sqljson.out | 98 +++++++++++++++++++++++++++++++++++
src/test/regress/sql/sqljson.sql      | 41 +++++++++++++++
3 files changed, 171 insertions(+)