Do not override combinefunc of custom aggregates with common names (#6805)

DESCRIPTION: Fix an issue that caused some queries with custom
aggregates to fail

While playing around with https://github.com/pgvector/pgvector I noticed
that the AVG query was broken. That's because we treat it as any other
AVG by breaking it down in SUM and COUNT, but there are no SUM/COUNT
functions in this case, but there is a perfectly usable combinefunc.
This PR changes our aggregate logic to prefer custom aggregates with a
combinefunc even if they have a common name.

Co-authored-by: Marco Slot <marco.slot@gmail.com>

src/backend/distributed/planner/multi_logical_optimizer.c

@@ -3385,6 +3385,13 @@ GetAggregateType(Aggref *aggregateExpression)
 {
 	Oid aggFunctionId = aggregateExpression->aggfnoid;
 
+	/* custom aggregates with combine func take precedence over name-based logic */
+	if (aggFunctionId >= FirstNormalObjectId &&
+		AggregateEnabledCustom(aggregateExpression))
+	{
+		return AGGREGATE_CUSTOM_COMBINE;
+	}
+
 	/* look up the function name */
 	char *aggregateProcName = get_func_name(aggFunctionId);
 	if (aggregateProcName == NULL)
@@ -3395,8 +3402,6 @@ GetAggregateType(Aggref *aggregateExpression)
 
 	uint32 aggregateCount = lengthof(AggregateNames);
 
-	Assert(AGGREGATE_INVALID_FIRST == 0);
-
 	for (uint32 aggregateIndex = 1; aggregateIndex < aggregateCount; aggregateIndex++)
 	{
 		const char *aggregateName = AggregateNames[aggregateIndex];
@@ -3465,7 +3470,7 @@ GetAggregateType(Aggref *aggregateExpression)
 		}
 	}
 
-
+	/* handle any remaining built-in aggregates with a suitable combinefn */
 	if (AggregateEnabledCustom(aggregateExpression))
 	{
 		return AGGREGATE_CUSTOM_COMBINE;

src/test/regress/expected/aggregate_support.out

@@ -665,6 +665,8 @@ select array_collect_sort(val) from aggdata;
 (1 row)
 
 reset role;
+drop owned by notsuper;
+drop user notsuper;
 -- Test aggregation on coordinator
 set citus.coordinator_aggregation_strategy to 'row-gather';
 select key, first(val order by id), last(val order by id)
@@ -1233,11 +1235,40 @@ CREATE AGGREGATE newavg (
    initcond1 = '{0,0}'
 );
 SELECT run_command_on_workers($$select aggfnoid from pg_aggregate where aggfnoid::text like '%newavg%';$$);
-               run_command_on_workers
+            run_command_on_workers
 ---------------------------------------------------------------------
  (localhost,57637,t,aggregate_support.newavg)
  (localhost,57638,t,aggregate_support.newavg)
 (2 rows)
 
+CREATE TYPE coord AS (x int, y int);
+CREATE FUNCTION coord_minx_sfunc(state coord, new coord)
+returns coord immutable language plpgsql as $$
+BEGIN
+   IF (state IS NULL OR new.x < state.x) THEN
+     RETURN new;
+   ELSE
+     RETURN state;
+   END IF;
+END
+$$;
+create function coord_minx_finalfunc(state coord)
+returns coord immutable language plpgsql as $$
+begin return state;
+end;
+$$;
+-- custom aggregate that has the same name as a built-in function, but with a combinefunc
+create aggregate min (coord) (
+    sfunc = coord_minx_sfunc,
+    stype = coord,
+    finalfunc = coord_minx_finalfunc,
+    combinefunc = coord_minx_sfunc
+);
+select min((id,val)::coord) from aggdata;
+  min
+---------------------------------------------------------------------
+ (1,2)
+(1 row)
+
 set client_min_messages to error;
 drop schema aggregate_support cascade;

src/test/regress/sql/aggregate_support.sql

@@ -364,6 +364,8 @@ $$);
 set role notsuper;
 select array_collect_sort(val) from aggdata;
 reset role;
+drop owned by notsuper;
+drop user notsuper;
 
 -- Test aggregation on coordinator
 set citus.coordinator_aggregation_strategy to 'row-gather';
@@ -645,5 +647,34 @@ CREATE AGGREGATE newavg (
 
 SELECT run_command_on_workers($$select aggfnoid from pg_aggregate where aggfnoid::text like '%newavg%';$$);
 
+CREATE TYPE coord AS (x int, y int);
+
+CREATE FUNCTION coord_minx_sfunc(state coord, new coord)
+returns coord immutable language plpgsql as $$
+BEGIN
+   IF (state IS NULL OR new.x < state.x) THEN
+     RETURN new;
+   ELSE
+     RETURN state;
+   END IF;
+END
+$$;
+
+create function coord_minx_finalfunc(state coord)
+returns coord immutable language plpgsql as $$
+begin return state;
+end;
+$$;
+
+-- custom aggregate that has the same name as a built-in function, but with a combinefunc
+create aggregate min (coord) (
+    sfunc = coord_minx_sfunc,
+    stype = coord,
+    finalfunc = coord_minx_finalfunc,
+    combinefunc = coord_minx_sfunc
+);
+
+select min((id,val)::coord) from aggdata;
+
 set client_min_messages to error;
 drop schema aggregate_support cascade;