Fix const-folding of JSON constructors inside a simple CASE

A JSON constructor with a RETURNING clause uses a CaseTestExpr as the
placeholder for its result in the coercion expression.  When such a
constructor appears in a WHEN clause of a simple CASE whose test
expression is a constant, eval_const_expressions substituted that
constant for the placeholder, so the coercion produced the CASE's test
value instead of the constructor's result.  For instance,

    CASE 'x' WHEN JSON_OBJECT('a': 'b' RETURNING text) THEN 1 ELSE 0 END

evaluated to 1.

To fix, keep case_val out of scope while simplifying the coercion, as
is already done for the elemexpr of an ArrayCoerceExpr.

Back-patch to v16, where the SQL/JSON constructor functions were
introduced.

Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAMbWs48A=VCFbteTkuCoknO1_0-Cu0aMBT0M07dm7vj1QyixDg@mail.gmail.com
Backpatch-through: 16

src/backend/optimizer/util/clauses.c

@@ -2904,7 +2904,6 @@ eval_const_expressions_mutator(Node *node,
 				}
 				break;
 			}
-
 		case T_JsonValueExpr:
 			{
 				JsonValueExpr *jve = (JsonValueExpr *) node;
@@ -2928,7 +2927,41 @@ eval_const_expressions_mutator(Node *node,
 												  (Expr *) formatted_expr,
 												  copyObject(jve->format));
 			}
+		case T_JsonConstructorExpr:
+			{
+				JsonConstructorExpr *jce = (JsonConstructorExpr *) node;
+				JsonConstructorExpr *newjce;
+				Node	   *save_case_val;
 
+				/*
+				 * Copy the node and const-simplify its arguments.  We can't
+				 * use ece_generic_processing() here because we need to mess
+				 * with case_val only while processing the coercion.
+				 */
+				newjce = makeNode(JsonConstructorExpr);
+				memcpy(newjce, jce, sizeof(JsonConstructorExpr));
+				newjce->args = (List *)
+					eval_const_expressions_mutator((Node *) jce->args,
+												   context);
+				newjce->func = (Expr *)
+					eval_const_expressions_mutator((Node *) jce->func,
+												   context);
+
+				/*
+				 * Set up for the CaseTestExpr node contained in the coercion.
+				 * We must prevent it from absorbing any outer CASE value.
+				 */
+				save_case_val = context->case_val;
+				context->case_val = NULL;
+
+				newjce->coercion = (Expr *)
+					eval_const_expressions_mutator((Node *) jce->coercion,
+												   context);
+
+				context->case_val = save_case_val;
+
+				return (Node *) newjce;
+			}
 		case T_SubPlan:
 		case T_AlternativeSubPlan:
 

src/include/nodes/primnodes.h

@@ -1271,11 +1271,11 @@ typedef struct CaseWhen
  *	* Placeholder for intermediate results in some SQL/JSON expression nodes,
  *	  such as JsonConstructorExpr.
  *
- * The uses in CaseExpr and ArrayCoerceExpr are safe only to the extent that
- * there is not any other CaseExpr or ArrayCoerceExpr between the value source
- * node and its child CaseTestExpr(s).  This is true in the parse analysis
- * output, but the planner's function-inlining logic has to be careful not to
- * break it.
+ * The uses in CaseExpr, ArrayCoerceExpr, and JsonConstructorExpr are safe
+ * only to the extent that there is not any other such node between the
+ * value source node and its child CaseTestExpr(s).  This is true in the
+ * parse analysis output, but the planner's constant-folding of simple CASE
+ * and its function-inlining logic have to be careful not to break it.
  *
  * The nested-assignment-expression case is safe because the only node types
  * that can be above such CaseTestExprs are FieldStore and SubscriptingRef.

src/test/regress/expected/sqljson.out

@@ -281,6 +281,13 @@ SELECT JSON_OBJECT(1: 1, '2': NULL, '3': 1, 4: NULL, '5': 'a' ABSENT ON NULL WIT
  {"1": 1, "3": 1, "5": "a"}
 (1 row)
 
+-- the RETURNING coercion must not pick up the test value of an enclosing CASE
+SELECT CASE 'x' WHEN JSON_OBJECT('a': 'b' RETURNING text) THEN 1 ELSE 0 END;
+ case 
+------
+    0
+(1 row)
+
 -- JSON_ARRAY()
 SELECT JSON_ARRAY();
  json_array 

src/test/regress/sql/sqljson.sql

@@ -88,6 +88,8 @@ SELECT JSON_OBJECT(1: 1, '2': NULL, '1': 1 ABSENT ON NULL WITH UNIQUE RETURNING
 SELECT JSON_OBJECT(1: 1, '2': NULL, '1': 1 ABSENT ON NULL WITHOUT UNIQUE RETURNING jsonb);
 SELECT JSON_OBJECT(1: 1, '2': NULL, '3': 1, 4: NULL, '5': 'a' ABSENT ON NULL WITH UNIQUE RETURNING jsonb);
 
+-- the RETURNING coercion must not pick up the test value of an enclosing CASE
+SELECT CASE 'x' WHEN JSON_OBJECT('a': 'b' RETURNING text) THEN 1 ELSE 0 END;
 
 -- JSON_ARRAY()
 SELECT JSON_ARRAY();