SQL/JSON: Fix some oversights in commit b6e1157e7

The decision in b6e1157e7 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

1 files changed, 9 insertions, 5 deletions

diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index 5b7b87dea54..b0ef1952e8f 100644
--- a/src/include/nodes/primnodes.h
+++ b/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;