Don't allow CTEs to determine semantic levels of aggregates.

The fix for bug #19055 (commit b0cc0a71e) allowed CTE references in
sub-selects within aggregate functions to affect the semantic levels
assigned to such aggregates.  It turns out this broke some related
cases, leading to assertion failures or strange planner errors such
as "unexpected outer reference in CTE query".  After experimenting
with some alternative rules for assigning the semantic level in
such cases, we've come to the conclusion that changing the level
is more likely to break things than be helpful.

Therefore, this patch undoes what b0cc0a71e changed, and instead
installs logic to throw an error if there is any reference to a
CTE that's below the semantic level that standard SQL rules would
assign to the aggregate based on its contained Var and Aggref nodes.
(The SQL standard disallows sub-selects within aggregate functions,
so it can't reach the troublesome case and hence has no rule for
what to do.)

Perhaps someone will come along with a legitimate query that this
logic rejects, and if so probably the example will help us craft
a level-adjustment rule that works better than what b0cc0a71e did.
I'm not holding my breath for that though, because the previous
logic had been there for a very long time before bug #19055 without
complaints, and that bug report sure looks to have originated from
fuzzing not from real usage.

Like b0cc0a71e, back-patch to all supported branches, though
sadly that no longer includes v13.

Bug: #19106
Reported-by: Kamil Monicz <kamil@monicz.dev>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19106-9dd3668a0734cd72@postgresql.org
Backpatch-through: 14

1 files changed, 38 insertions, 11 deletions

diff --git a/src/backend/parser/parse_agg.c b/src/backend/parser/parse_agg.c
index 3254c83cc6c..b8340557b34 100644
--- a/src/backend/parser/parse_agg.c
+++ b/src/backend/parser/parse_agg.c
@@ -38,6 +38,8 @@ typedef struct
 	ParseState *pstate;
 	int			min_varlevel;
 	int			min_agglevel;
+	int			min_ctelevel;
+	RangeTblEntry *min_cte;
 	int			sublevels_up;
 } check_agg_arguments_context;
 
@@ -58,7 +60,8 @@ typedef struct
 static int	check_agg_arguments(ParseState *pstate,
 								List *directargs,
 								List *args,
-								Expr *filter);
+								Expr *filter,
+								int agglocation);
 static bool check_agg_arguments_walker(Node *node,
 									   check_agg_arguments_context *context);
 static Node *substitute_grouped_columns(Node *node, ParseState *pstate, Query *qry,
@@ -339,7 +342,8 @@ check_agglevels_and_constraints(ParseState *pstate, Node *expr)
 	min_varlevel = check_agg_arguments(pstate,
 									   directargs,
 									   args,
-									   filter);
+									   filter,
+									   location);
 
 	*p_levelsup = min_varlevel;
 
@@ -641,7 +645,8 @@ static int
 check_agg_arguments(ParseState *pstate,
 					List *directargs,
 					List *args,
-					Expr *filter)
+					Expr *filter,
+					int agglocation)
 {
 	int			agglevel;
 	check_agg_arguments_context context;
@@ -649,6 +654,8 @@ check_agg_arguments(ParseState *pstate,
 	context.pstate = pstate;
 	context.min_varlevel = -1;	/* signifies nothing found yet */
 	context.min_agglevel = -1;
+	context.min_ctelevel = -1;
+	context.min_cte = NULL;
 	context.sublevels_up = 0;
 
 	(void) check_agg_arguments_walker((Node *) args, &context);
@@ -687,6 +694,20 @@ check_agg_arguments(ParseState *pstate,
 	}
 
 	/*
+	 * If there's a non-local CTE that's below the aggregate's semantic level,
+	 * complain.  It's not quite clear what we should do to fix up such a case
+	 * (treating the CTE reference like a Var seems wrong), and it's also
+	 * unclear whether there is a real-world use for such cases.
+	 */
+	if (context.min_ctelevel >= 0 && context.min_ctelevel < agglevel)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("outer-level aggregate cannot use a nested CTE"),
+				 errdetail("CTE \"%s\" is below the aggregate's semantic level.",
+						   context.min_cte->eref->aliasname),
+				 parser_errposition(pstate, agglocation)));
+
+	/*
 	 * Now check for vars/aggs in the direct arguments, and throw error if
 	 * needed.  Note that we allow a Var of the agg's semantic level, but not
 	 * an Agg of that level.  In principle such Aggs could probably be
@@ -699,6 +720,7 @@ check_agg_arguments(ParseState *pstate,
 	{
 		context.min_varlevel = -1;
 		context.min_agglevel = -1;
+		context.min_ctelevel = -1;
 		(void) check_agg_arguments_walker((Node *) directargs, &context);
 		if (context.min_varlevel >= 0 && context.min_varlevel < agglevel)
 			ereport(ERROR,
@@ -714,6 +736,13 @@ check_agg_arguments(ParseState *pstate,
 					 parser_errposition(pstate,
 										locate_agg_of_level((Node *) directargs,
 															context.min_agglevel))));
+		if (context.min_ctelevel >= 0 && context.min_ctelevel < agglevel)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("outer-level aggregate cannot use a nested CTE"),
+					 errdetail("CTE \"%s\" is below the aggregate's semantic level.",
+							   context.min_cte->eref->aliasname),
+					 parser_errposition(pstate, agglocation)));
 	}
 	return agglevel;
 }
@@ -794,11 +823,6 @@ check_agg_arguments_walker(Node *node,
 
 	if (IsA(node, RangeTblEntry))
 	{
-		/*
-		 * CTE references act similarly to Vars of the CTE's level.  Without
-		 * this we might conclude that the Agg can be evaluated above the CTE,
-		 * leading to trouble.
-		 */
 		RangeTblEntry *rte = (RangeTblEntry *) node;
 
 		if (rte->rtekind == RTE_CTE)
@@ -810,9 +834,12 @@ check_agg_arguments_walker(Node *node,
 			/* ignore local CTEs of subqueries */
 			if (ctelevelsup >= 0)
 			{
-				if (context->min_varlevel < 0 ||
-					context->min_varlevel > ctelevelsup)
-					context->min_varlevel = ctelevelsup;
+				if (context->min_ctelevel < 0 ||
+					context->min_ctelevel > ctelevelsup)
+				{
+					context->min_ctelevel = ctelevelsup;
+					context->min_cte = rte;
+				}
 			}
 		}
 		return false;			/* allow range_table_walker to continue */