Fix issue with ORDER BY / DISTINCT aggregates and FILTER

david-rowley · david-rowley · commit 065ce49a10e0 · 2025-04-20T22:12:37.000+12:00
1349d27 added support so that aggregate functions with an ORDER BY or DISTINCT clause could make use of presorted inputs to avoid an implicit sort within nodeAgg.c. That commit failed to consider that a FILTER clause may exist that filters rows before the aggregate function arguments are evaluated. That can be problematic if an aggregate argument contains an expression which could error out during evaluation. It's perfectly valid to want to have a FILTER clause which eliminates such values, and with the pre-sorted path added in 1349d27, it was possible that the planner would produce a plan with a Sort node above the Aggregate to perform the sort on the aggregate's arguments long before the Aggregate node would filter out the non-matching values. Here we fix this by inspecting ORDER BY / DISTINCT aggregate functions which have a FILTER clause to see if the aggregate's arguments are anything more complex than a Var or a Const. Evaluating these isn't going to cause an error. If we find any non-Var, non-Const parameters then the planner will now opt to perform the sort in the Aggregate node for these aggregates, i.e. disable the presorted aggregate optimization. An alternative fix would have been to completely disallow the presorted optimization for Aggrefs with any FILTER clause, but that wasn't done as that could cause large performance regressions for queries that see significant gains from 1349d27 due to presorted results coming in from an Index Scan. Backpatch to 16, where 1349d27 was introduced Author: David Rowley <dgrowleyml@gmail.com> Reported-by: Kaimeh <kkaimeh@gmail.com> Diagnosed-by: Tom Lane <tgl@sss.pgh.pa.us> Discussion: https://postgr.es/m/CAK-%2BJz9J%3DQ06-M7cDJoPNeYbz5EZDqkjQbJnmRyQyzkbRGsYkA%40mail.gmail.com Backpatch-through: 16
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
@@ -3280,10 +3280,53 @@ adjust_group_pathkeys_for_groupagg(PlannerInfo *root)
 		if (AGGKIND_IS_ORDERED_SET(aggref->aggkind))
 			continue;
 
-		/* only add aggregates with a DISTINCT or ORDER BY */
-		if (aggref->aggdistinct != NIL || aggref->aggorder != NIL)
-			unprocessed_aggs = bms_add_member(unprocessed_aggs,
-											  foreach_current_index(lc));
+		/* Skip unless there's a DISTINCT or ORDER BY clause */
+		if (aggref->aggdistinct == NIL && aggref->aggorder == NIL)
+			continue;
+
+		/* Additional safety checks are needed if there's a FILTER clause */
+		if (aggref->aggfilter != NULL)
+		{
+			ListCell   *lc2;
+			bool		allow_presort = true;
+
+			/*
+			 * When the Aggref has a FILTER clause, it's possible that the
+			 * filter removes rows that cannot be sorted because the
+			 * expression to sort by results in an error during its
+			 * evaluation.  This is a problem for presorting as that happens
+			 * before the FILTER, whereas without presorting, the Aggregate
+			 * node will apply the FILTER *before* sorting.  So that we never
+			 * try to sort anything that might error, here we aim to skip over
+			 * any Aggrefs with arguments with expressions which, when
+			 * evaluated, could cause an ERROR.  Vars and Consts are ok. There
+			 * may be more cases that should be allowed, but more thought
+			 * needs to be given.  Err on the side of caution.
+			 */
+			foreach(lc2, aggref->args)
+			{
+				TargetEntry *tle = (TargetEntry *) lfirst(lc2);
+				Expr	   *expr = tle->expr;
+
+				while (IsA(expr, RelabelType))
+					expr = (Expr *) (castNode(RelabelType, expr))->arg;
+
+				/* Common case, Vars and Consts are ok */
+				if (IsA(expr, Var) || IsA(expr, Const))
+					continue;
+
+				/* Unsupported.  Don't try to presort for this Aggref */
+				allow_presort = false;
+				break;
+			}
+
+			/* Skip unsupported Aggrefs */
+			if (!allow_presort)
+				continue;
+		}
+
+		unprocessed_aggs = bms_add_member(unprocessed_aggs,
+										  foreach_current_index(lc));
 	}
 
 	/*
diff --git a/src/test/regress/expected/aggregates.out b/src/test/regress/expected/aggregates.out
@@ -1586,6 +1586,43 @@ select sum(two order by two) from tenk1;
 (2 rows)
 
 reset enable_presorted_aggregate;
+--
+-- Test cases with FILTER clause
+--
+-- Ensure we presort when the aggregate contains plain Vars
+explain (costs off)
+select sum(two order by two) filter (where two > 1) from tenk1;
+          QUERY PLAN           
+-------------------------------
+ Aggregate
+   ->  Sort
+         Sort Key: two
+         ->  Seq Scan on tenk1
+(4 rows)
+
+-- Ensure we presort for RelabelType'd Vars
+explain (costs off)
+select string_agg(distinct f1, ',') filter (where length(f1) > 1)
+from varchar_tbl;
+             QUERY PLAN              
+-------------------------------------
+ Aggregate
+   ->  Sort
+         Sort Key: f1
+         ->  Seq Scan on varchar_tbl
+(4 rows)
+
+-- Ensure we don't presort when the aggregate's argument contains an
+-- explicit cast.
+explain (costs off)
+select string_agg(distinct f1::varchar(2), ',') filter (where length(f1) > 1)
+from varchar_tbl;
+          QUERY PLAN           
+-------------------------------
+ Aggregate
+   ->  Seq Scan on varchar_tbl
+(2 rows)
+
 --
 -- Test combinations of DISTINCT and/or ORDER BY
 --
diff --git a/src/test/regress/expected/sqljson.out b/src/test/regress/expected/sqljson.out
@@ -835,22 +835,22 @@ SELECT
 FROM
 	(VALUES (NULL), (3), (1), (NULL), (NULL), (5), (2), (4), (NULL)) foo(bar);
 -[ RECORD 1 ]--------------------+-------------------------------------------------------------------------------------------------------------------------
-no_options                       | [1, 2, 3, 4, 5]
-returning_jsonb                  | [1, 2, 3, 4, 5]
-absent_on_null                   | [1, 2, 3, 4, 5]
-absentonnull_returning_jsonb     | [1, 2, 3, 4, 5]
-null_on_null                     | [1, 2, 3, 4, 5, null, null, null, null]
-nullonnull_returning_jsonb       | [1, 2, 3, 4, 5, null, null, null, null]
-row_no_options                   | [{"bar":1},                                                                                                             +
-                                 |  {"bar":2},                                                                                                             +
+no_options                       | [3, 1, 5, 2, 4]
+returning_jsonb                  | [3, 1, 5, 2, 4]
+absent_on_null                   | [3, 1, 5, 2, 4]
+absentonnull_returning_jsonb     | [3, 1, 5, 2, 4]
+null_on_null                     | [null, 3, 1, null, null, 5, 2, 4, null]
+nullonnull_returning_jsonb       | [null, 3, 1, null, null, 5, 2, 4, null]
+row_no_options                   | [{"bar":null},                                                                                                          +
                                  |  {"bar":3},                                                                                                             +
-                                 |  {"bar":4},                                                                                                             +
-                                 |  {"bar":5},                                                                                                             +
-                                 |  {"bar":null},                                                                                                          +
+                                 |  {"bar":1},                                                                                                             +
                                  |  {"bar":null},                                                                                                          +
                                  |  {"bar":null},                                                                                                          +
+                                 |  {"bar":5},                                                                                                             +
+                                 |  {"bar":2},                                                                                                             +
+                                 |  {"bar":4},                                                                                                             +
                                  |  {"bar":null}]
-row_returning_jsonb              | [{"bar": 1}, {"bar": 2}, {"bar": 3}, {"bar": 4}, {"bar": 5}, {"bar": null}, {"bar": null}, {"bar": null}, {"bar": null}]
+row_returning_jsonb              | [{"bar": null}, {"bar": 3}, {"bar": 1}, {"bar": null}, {"bar": null}, {"bar": 5}, {"bar": 2}, {"bar": 4}, {"bar": null}]
 row_filtered_agg                 | [{"bar":3},                                                                                                             +
                                  |  {"bar":4},                                                                                                             +
                                  |  {"bar":5}]
diff --git a/src/test/regress/sql/aggregates.sql b/src/test/regress/sql/aggregates.sql
@@ -595,6 +595,25 @@ explain (costs off)
 select sum(two order by two) from tenk1;
 reset enable_presorted_aggregate;
 
+--
+-- Test cases with FILTER clause
+--
+
+-- Ensure we presort when the aggregate contains plain Vars
+explain (costs off)
+select sum(two order by two) filter (where two > 1) from tenk1;
+
+-- Ensure we presort for RelabelType'd Vars
+explain (costs off)
+select string_agg(distinct f1, ',') filter (where length(f1) > 1)
+from varchar_tbl;
+
+-- Ensure we don't presort when the aggregate's argument contains an
+-- explicit cast.
+explain (costs off)
+select string_agg(distinct f1::varchar(2), ',') filter (where length(f1) > 1)
+from varchar_tbl;
+
 --
 -- Test combinations of DISTINCT and/or ORDER BY
 --
