Suppress unnecessary RelabelType nodes in yet more cases.

Commit a477bfc1d fixed eval_const_expressions() to ensure that it
didn't generate unnecessary RelabelType nodes, but I failed to notice
that some other places in the planner had the same issue.  Really
noplace in the planner should be using plain makeRelabelType(), for
fear of generating expressions that should be equal() to semantically
equivalent trees, but aren't.

An example is that because canonicalize_ec_expression() failed
to be careful about this, we could end up with an equivalence class
containing both a plain Const, and a Const-with-RelabelType
representing exactly the same value.  So far as I can tell this led to
no visible misbehavior, but we did waste a bunch of cycles generating
and evaluating "Const = Const-with-RelabelType" to prove such entries
are redundant.

Hence, move the support function added by a477bfc1d to where it can
be more generally useful, and use it in the places where planner code
previously used makeRelabelType.

Back-patch to v12, like the previous patch.  While I have no concrete
evidence of any real misbehavior here, it's certainly possible that
I overlooked a case where equivalent expressions that aren't equal()
could cause a user-visible problem.  In any case carrying extra
RelabelType nodes through planning to execution isn't very desirable.

Discussion: https://postgr.es/m/1311836.1597781384@sss.pgh.pa.us

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);
 }
 
 /*