SQL/JSON: Fix some oversights in commit b6e1157

The decision in b6e1157 to ignore raw_expr when evaluating a
JsonValueExpr was incorrect.  While its value is not ultimately
used (since formatted_expr's value is), failing to initialize it
can lead to problems, for instance,  when the expression tree in
raw_expr contains Aggref nodes, which must be initialized to
ensure the parent Agg node works correctly.

Also, optimize eval_const_expressions_mutator()'s handling of
JsonValueExpr a bit.  Currently, when formatted_expr cannot be folded
into a constant, we end up processing it twice -- once directly in
eval_const_expressions_mutator() and again recursively via
ece_generic_processing().  This recursive processing is required to
handle raw_expr. To avoid the redundant processing of formatted_expr,
we now  process raw_expr directly in eval_const_expressions_mutator().

Finally, update the comment of JsonValueExpr to describe the roles of
raw_expr and formatted_expr more clearly.

Bug: #18657
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Diagnosed-by: Fabio R. Sluzala <fabio3rs@gmail.com>
Diagnosed-by: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/18657-1b90ccce2b16bdb8@postgresql.org
Backpatch-through: 16

Author: amitlan

src/backend/executor/execExpr.c

@@ -2307,6 +2307,8 @@ ExecInitExprRec(Expr *node, ExprState *state,
 			{
 				JsonValueExpr *jve = (JsonValueExpr *) node;
 
+				Assert(jve->raw_expr != NULL);
+				ExecInitExprRec(jve->raw_expr, state, resv, resnull);
 				Assert(jve->formatted_expr != NULL);
 				ExecInitExprRec(jve->formatted_expr, state, resv, resnull);
 				break;

src/backend/optimizer/util/clauses.c

@@ -2915,13 +2915,25 @@ eval_const_expressions_mutator(Node *node,
 		case T_JsonValueExpr:
 			{
 				JsonValueExpr *jve = (JsonValueExpr *) node;
-				Node	   *formatted;
+				Node	   *raw_expr = (Node *) jve->raw_expr;
+				Node	   *formatted_expr = (Node *) jve->formatted_expr;
 
-				formatted = eval_const_expressions_mutator((Node *) jve->formatted_expr,
-														   context);
-				if (formatted && IsA(formatted, Const))
-					return formatted;
-				break;
+				/*
+				 * If we can fold formatted_expr to a constant, we can elide
+				 * the JsonValueExpr altogether.  Otherwise we must process
+				 * raw_expr too.  But JsonFormat is a flat node and requires
+				 * no simplification, only copying.
+				 */
+				formatted_expr = eval_const_expressions_mutator(formatted_expr,
+																context);
+				if (formatted_expr && IsA(formatted_expr, Const))
+					return formatted_expr;
+
+				raw_expr = eval_const_expressions_mutator(raw_expr, context);
+
+				return (Node *) makeJsonValueExpr((Expr *) raw_expr,
+												  (Expr *) formatted_expr,
+												  copyObject(jve->format));
 			}
 
 		case T_SubPlan:

src/include/nodes/primnodes.h

@@ -1669,15 +1669,19 @@ typedef struct JsonReturning
  * JsonValueExpr -
  *		representation of JSON value expression (expr [FORMAT JsonFormat])
  *
- * The actual value is obtained by evaluating formatted_expr.  raw_expr is
- * only there for displaying the original user-written expression and is not
- * evaluated by ExecInterpExpr() and eval_const_expressions_mutator().
+ * raw_expr is the user-specified value, while formatted_expr is the value
+ * obtained by coercing raw_expr to the type required by either the FORMAT
+ * clause or an enclosing node's RETURNING clause.
+ *
+ * When deparsing a JsonValueExpr, get_rule_expr() prints raw_expr. However,
+ * during the evaluation of a JsonValueExpr, the value of formatted_expr
+ * takes precedence over that of raw_expr.
  */
 typedef struct JsonValueExpr
 {
 	NodeTag		type;
-	Expr	   *raw_expr;		/* raw expression */
-	Expr	   *formatted_expr; /* formatted expression */
+	Expr	   *raw_expr;		/* user-specified expression */
+	Expr	   *formatted_expr; /* coerced formatted expression */
 	JsonFormat *format;			/* FORMAT clause, if specified */
 } JsonValueExpr;
 

src/test/regress/expected/sqljson.out

@@ -1300,3 +1300,52 @@ SELECT JSON_SERIALIZE('123' RETURNING sqljson_char2);
 ERROR:  value too long for type character(2)
 SELECT JSON_SERIALIZE('12' RETURNING sqljson_char2);
 ERROR:  value for domain sqljson_char2 violates check constraint "sqljson_char2_check"
+-- Bug #18657: JsonValueExpr.raw_expr was not initialized in ExecInitExprRec()
+-- causing the Aggrefs contained in it to also not be initialized, which led
+-- to a crash in ExecBuildAggTrans() as mentioned in the bug report:
+-- https://postgr.es/m/18657-1b90ccce2b16bdb8@postgresql.org
+CREATE FUNCTION volatile_one() RETURNS int AS $$ BEGIN RETURN 1; END; $$ LANGUAGE plpgsql VOLATILE;
+CREATE FUNCTION stable_one() RETURNS int AS $$ BEGIN RETURN 1; END; $$ LANGUAGE plpgsql STABLE;
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': volatile_one() RETURNING text) FORMAT JSON);
+                                                 QUERY PLAN                                                  
+-------------------------------------------------------------------------------------------------------------
+ Aggregate
+   Output: JSON_OBJECT('a' : JSON_OBJECTAGG('b' : volatile_one() RETURNING text) FORMAT JSON RETURNING json)
+   ->  Result
+(3 rows)
+
+SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': volatile_one() RETURNING text) FORMAT JSON);
+     json_object     
+---------------------
+ {"a" : { "b" : 1 }}
+(1 row)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': stable_one() RETURNING text) FORMAT JSON);
+                                                QUERY PLAN                                                 
+-----------------------------------------------------------------------------------------------------------
+ Aggregate
+   Output: JSON_OBJECT('a' : JSON_OBJECTAGG('b' : stable_one() RETURNING text) FORMAT JSON RETURNING json)
+   ->  Result
+(3 rows)
+
+SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': stable_one() RETURNING text) FORMAT JSON);
+     json_object     
+---------------------
+ {"a" : { "b" : 1 }}
+(1 row)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': 1 RETURNING text) FORMAT JSON);
+                                           QUERY PLAN                                           
+------------------------------------------------------------------------------------------------
+ Aggregate
+   Output: JSON_OBJECT('a' : JSON_OBJECTAGG('b' : 1 RETURNING text) FORMAT JSON RETURNING json)
+   ->  Result
+(3 rows)
+
+SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': 1 RETURNING text) FORMAT JSON);
+     json_object     
+---------------------
+ {"a" : { "b" : 1 }}
+(1 row)
+
+DROP FUNCTION volatile_one, stable_one;

src/test/regress/sql/sqljson.sql

@@ -480,3 +480,17 @@ SELECT JSON_OBJECTAGG(i: ('111' || i)::bytea FORMAT JSON WITH UNIQUE RETURNING v
 CREATE DOMAIN sqljson_char2 AS char(2) CHECK (VALUE NOT IN ('12'));
 SELECT JSON_SERIALIZE('123' RETURNING sqljson_char2);
 SELECT JSON_SERIALIZE('12' RETURNING sqljson_char2);
+
+-- Bug #18657: JsonValueExpr.raw_expr was not initialized in ExecInitExprRec()
+-- causing the Aggrefs contained in it to also not be initialized, which led
+-- to a crash in ExecBuildAggTrans() as mentioned in the bug report:
+-- https://postgr.es/m/18657-1b90ccce2b16bdb8@postgresql.org
+CREATE FUNCTION volatile_one() RETURNS int AS $$ BEGIN RETURN 1; END; $$ LANGUAGE plpgsql VOLATILE;
+CREATE FUNCTION stable_one() RETURNS int AS $$ BEGIN RETURN 1; END; $$ LANGUAGE plpgsql STABLE;
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': volatile_one() RETURNING text) FORMAT JSON);
+SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': volatile_one() RETURNING text) FORMAT JSON);
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': stable_one() RETURNING text) FORMAT JSON);
+SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': stable_one() RETURNING text) FORMAT JSON);
+EXPLAIN (VERBOSE, COSTS OFF) SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': 1 RETURNING text) FORMAT JSON);
+SELECT JSON_OBJECT('a': JSON_OBJECTAGG('b': 1 RETURNING text) FORMAT JSON);
+DROP FUNCTION volatile_one, stable_one;