1 files changed, 62 insertions, 13 deletions

diff --git a/src/backend/nodes/nodeFuncs.c b/src/backend/nodes/nodeFuncs.c
index 6924a4920ef..64c7eca95b0 100644
--- a/src/backend/nodes/nodeFuncs.c
+++ b/src/backend/nodes/nodeFuncs.c
@@ -577,26 +577,75 @@ exprIsLengthCoercion(const Node *expr, int32 *coercedTypmod)
 }
 
 /*
+ * applyRelabelType
+ *		Add a RelabelType node if needed to make the expression expose
+ *		the specified type, typmod, and collation.
+ *
+ * This is primarily intended to be used during planning.  Therefore, it must
+ * maintain the post-eval_const_expressions invariants that there are not
+ * adjacent RelabelTypes, and that the tree is fully const-folded (hence,
+ * we mustn't return a RelabelType atop a Const).  If we do find a Const,
+ * we'll modify it in-place if "overwrite_ok" is true; that should only be
+ * passed as true if caller knows the Const is newly generated.
+ */
+Node *
+applyRelabelType(Node *arg, Oid rtype, int32 rtypmod, Oid rcollid,
+				 CoercionForm rformat, int rlocation, bool overwrite_ok)
+{
+	/*
+	 * If we find stacked RelabelTypes (eg, from foo::int::oid) we can discard
+	 * all but the top one, and must do so to ensure that semantically
+	 * equivalent expressions are equal().
+	 */
+	while (arg && IsA(arg, RelabelType))
+		arg = (Node *) ((RelabelType *) arg)->arg;
+
+	if (arg && IsA(arg, Const))
+	{
+		/* Modify the Const directly to preserve const-flatness. */
+		Const	   *con = (Const *) arg;
+
+		if (!overwrite_ok)
+			con = copyObject(con);
+		con->consttype = rtype;
+		con->consttypmod = rtypmod;
+		con->constcollid = rcollid;
+		/* We keep the Const's original location. */
+		return (Node *) con;
+	}
+	else if (exprType(arg) == rtype &&
+			 exprTypmod(arg) == rtypmod &&
+			 exprCollation(arg) == rcollid)
+	{
+		/* Sometimes we find a nest of relabels that net out to nothing. */
+		return arg;
+	}
+	else
+	{
+		/* Nope, gotta have a RelabelType. */
+		RelabelType *newrelabel = makeNode(RelabelType);
+
+		newrelabel->arg = (Expr *) arg;
+		newrelabel->resulttype = rtype;
+		newrelabel->resulttypmod = rtypmod;
+		newrelabel->resultcollid = rcollid;
+		newrelabel->relabelformat = rformat;
+		newrelabel->location = rlocation;
+		return (Node *) newrelabel;
+	}
+}
+
+/*
  * relabel_to_typmod
  *		Add a RelabelType node that changes just the typmod of the expression.
  *
- * This is primarily intended to be used during planning.  Therefore, it
- * strips any existing RelabelType nodes to maintain the planner's invariant
- * that there are not adjacent RelabelTypes.
+ * Convenience function for a common usage of applyRelabelType.
  */
 Node *
 relabel_to_typmod(Node *expr, int32 typmod)
 {
-	Oid			type = exprType(expr);
-	Oid			coll = exprCollation(expr);
-
-	/* Strip any existing RelabelType node(s) */
-	while (expr && IsA(expr, RelabelType))
-		expr = (Node *) ((RelabelType *) expr)->arg;
-
-	/* Apply new typmod, preserving the previous exposed type and collation */
-	return (Node *) makeRelabelType((Expr *) expr, type, typmod, coll,
-									COERCE_EXPLICIT_CAST);
+	return applyRelabelType(expr, exprType(expr), typmod, exprCollation(expr),
+							COERCE_EXPLICIT_CAST, -1, false);
 }
 
 /*