Code review for protransform patches.

Fix loss of previous expression-simplification work when a transform
function fires: we must not simply revert to untransformed input tree.
Instead build a dummy FuncExpr node to pass to the transform function.
This has the additional advantage of providing a simpler, more uniform
API for transform functions.

Move documentation to a somewhat less buried spot, relocate some
poorly-placed code, be more wary of null constants and invalid typmod
values, add an opr_sanity check on protransform function signatures,
and some other minor cosmetic adjustments.

Note: although this patch touches pg_proc.h, no need for catversion
bump, because the changes are cosmetic and don't actually change the
intended catalog contents.

src/backend/nodes/nodeFuncs.c

@@ -17,6 +17,7 @@
 #include "catalog/pg_collation.h"
 #include "catalog/pg_type.h"
 #include "miscadmin.h"
+#include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
 #include "nodes/relation.h"
 #include "utils/builtins.h"
@@ -547,6 +548,30 @@ exprIsLengthCoercion(const Node *expr, int32 *coercedTypmod)
 	return false;
 }
 
+/*
+ * 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, and it uses COERCE_DONTCARE
+ * which would typically be inappropriate earlier.
+ */
+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_DONTCARE);
+}
+
 /*
  * expression_returns_set
  *	  Test whether an expression returns a set result.
@@ -2694,7 +2719,9 @@ query_or_expression_tree_mutator(Node *node,
  * that could appear under it, but not other statement types.
  */
 bool
-			raw_expression_tree_walker(Node *node, bool (*walker) (), void *context)
+raw_expression_tree_walker(Node *node,
+						   bool (*walker) (),
+						   void *context)
 {
 	ListCell   *temp;
 

src/backend/optimizer/util/clauses.c

@@ -107,11 +107,11 @@ static List *simplify_and_arguments(List *args,
 					   eval_const_expressions_context *context,
 					   bool *haveNull, bool *forceFalse);
 static Node *simplify_boolean_equality(Oid opno, List *args);
-static Expr *simplify_function(Expr *oldexpr, Oid funcid,
-				  Oid result_type, int32 result_typmod, Oid result_collid,
-				  Oid input_collid, List **args,
+static Expr *simplify_function(Oid funcid,
+				  Oid result_type, int32 result_typmod,
+				  Oid result_collid, Oid input_collid, List **args,
 				  bool has_named_args,
-				  bool allow_inline,
+				  bool allow_non_const,
 				  eval_const_expressions_context *context);
 static List *reorder_function_arguments(List *args, Oid result_type,
 						   HeapTuple func_tuple,
@@ -2332,8 +2332,7 @@ eval_const_expressions_mutator(Node *node,
 				 * length coercion; we want to preserve the typmod in the
 				 * eventual Const if so.
 				 */
-				simple = simplify_function((Expr *) expr,
-										   expr->funcid,
+				simple = simplify_function(expr->funcid,
 										   expr->funcresulttype,
 										   exprTypmod(node),
 										   expr->funccollid,
@@ -2389,8 +2388,7 @@ eval_const_expressions_mutator(Node *node,
 				 * Code for op/func reduction is pretty bulky, so split it out
 				 * as a separate function.
 				 */
-				simple = simplify_function((Expr *) expr,
-										   expr->opfuncid,
+				simple = simplify_function(expr->opfuncid,
 										   expr->opresulttype, -1,
 										   expr->opcollid,
 										   expr->inputcollid,
@@ -2491,8 +2489,7 @@ eval_const_expressions_mutator(Node *node,
 					 * Code for op/func reduction is pretty bulky, so split it
 					 * out as a separate function.
 					 */
-					simple = simplify_function((Expr *) expr,
-											   expr->opfuncid,
+					simple = simplify_function(expr->opfuncid,
 											   expr->opresulttype, -1,
 											   expr->opcollid,
 											   expr->inputcollid,
@@ -2698,8 +2695,7 @@ eval_const_expressions_mutator(Node *node,
 				getTypeInputInfo(expr->resulttype,
 								 &infunc, &intypioparam);
 
-				simple = simplify_function(NULL,
-										   outfunc,
+				simple = simplify_function(outfunc,
 										   CSTRINGOID, -1,
 										   InvalidOid,
 										   InvalidOid,
@@ -2728,8 +2724,7 @@ eval_const_expressions_mutator(Node *node,
 												false,
 												true));
 
-					simple = simplify_function(NULL,
-											   infunc,
+					simple = simplify_function(infunc,
 											   expr->resulttype, -1,
 											   expr->resultcollid,
 											   InvalidOid,
@@ -3581,15 +3576,11 @@ simplify_boolean_equality(Oid opno, List *args)
  * Subroutine for eval_const_expressions: try to simplify a function call
  * (which might originally have been an operator; we don't care)
  *
- * Inputs are the original expression (can be NULL), function OID, actual
- * result type OID (which is needed for polymorphic functions), result typmod,
- * result collation, the input collation to use for the function, the
- * pre-simplified argument list, and some flags; also the context data for
- * eval_const_expressions.	In common cases, several of the arguments could be
- * derived from the original expression.  Sending them separately avoids
- * duplicating NodeTag-specific knowledge, and it's necessary for CoerceViaIO.
- * A NULL original expression disables use of transform functions while
- * retaining all other behaviors.
+ * Inputs are the function OID, actual result type OID (which is needed for
+ * polymorphic functions), result typmod, result collation,
+ * the input collation to use for the function,
+ * the pre-simplified argument list, and some flags;
+ * also the context data for eval_const_expressions.
  *
  * Returns a simplified expression if successful, or NULL if cannot
  * simplify the function call.
@@ -3601,28 +3592,32 @@ simplify_boolean_equality(Oid opno, List *args)
  * pass-by-reference, and it may get modified even if simplification fails.
  */
 static Expr *
-simplify_function(Expr *oldexpr, Oid funcid,
-				  Oid result_type, int32 result_typmod, Oid result_collid,
-				  Oid input_collid, List **args,
+simplify_function(Oid funcid, Oid result_type, int32 result_typmod,
+				  Oid result_collid, Oid input_collid, List **args,
 				  bool has_named_args,
-				  bool allow_inline,
+				  bool allow_non_const,
 				  eval_const_expressions_context *context)
 {
 	HeapTuple	func_tuple;
+	Form_pg_proc func_form;
 	Expr	   *newexpr;
-	Oid			transform;
 
 	/*
 	 * We have three strategies for simplification: execute the function to
 	 * deliver a constant result, use a transform function to generate a
 	 * substitute node tree, or expand in-line the body of the function
 	 * definition (which only works for simple SQL-language functions, but
-	 * that is a common case).	Each needs access to the function's pg_proc
-	 * tuple, so fetch it just once.
+	 * that is a common case).  Each case needs access to the function's
+	 * pg_proc tuple, so fetch it just once.
+	 *
+	 * Note: the allow_non_const flag suppresses both the second and third
+	 * strategies; so if !allow_non_const, simplify_function can only return
+	 * a Const or NULL.  Argument-list rewriting happens anyway, though.
 	 */
 	func_tuple = SearchSysCache1(PROCOID, ObjectIdGetDatum(funcid));
 	if (!HeapTupleIsValid(func_tuple))
 		elog(ERROR, "cache lookup failed for function %u", funcid);
+	func_form = (Form_pg_proc) GETSTRUCT(func_tuple);
 
 	/*
 	 * While we have the tuple, reorder named arguments and add default
@@ -3631,48 +3626,38 @@ simplify_function(Expr *oldexpr, Oid funcid,
 	if (has_named_args)
 		*args = reorder_function_arguments(*args, result_type, func_tuple,
 										   context);
-	else if (((Form_pg_proc) GETSTRUCT(func_tuple))->pronargs > list_length(*args))
+	else if (func_form->pronargs > list_length(*args))
 		*args = add_function_defaults(*args, result_type, func_tuple, context);
 
 	newexpr = evaluate_function(funcid, result_type, result_typmod,
 								result_collid, input_collid, *args,
 								func_tuple, context);
 
-	/*
-	 * Some functions calls can be simplified at plan time based on properties
-	 * specific to the function.  For example, "varchar(s::varchar(4), 8,
-	 * true)" simplifies to "s::varchar(4)", and "int4mul(n, 1)" could
-	 * simplify to "n".  To define such function-specific optimizations, write
-	 * a "transform function" and store its OID in the pg_proc.protransform of
-	 * the primary function.  Give each transform function the signature
-	 * "protransform(internal) RETURNS internal".  The argument, internally an
-	 * Expr *, is the node representing a call to the primary function.  If
-	 * the transform function's study of that node proves that a simplified
-	 * Expr substitutes for all possible concrete calls represented thereby,
-	 * return that simplified Expr.  Otherwise, return the NULL pointer.
-	 *
-	 * Currently, the specific Expr nodetag can be FuncExpr, OpExpr or
-	 * DistinctExpr.  This list may change in the future.  The function should
-	 * check the nodetag and return the NULL pointer for unexpected inputs.
-	 *
-	 * We make no guarantee that PostgreSQL will never call the primary
-	 * function in cases that the transform function would simplify.  Ensure
-	 * rigorous equivalence between the simplified expression and an actual
-	 * call to the primary function.
-	 *
-	 * Currently, this facility is undocumented and not exposed to users at
-	 * the SQL level.  Core length coercion casts use it to avoid calls
-	 * guaranteed to return their input unchanged.	This in turn allows ALTER
-	 * TABLE ALTER TYPE to avoid rewriting tables for some typmod changes.	In
-	 * the future, this facility may find other applications, like simplifying
-	 * x*0, x*1, and x+0.
-	 */
-	transform = ((Form_pg_proc) GETSTRUCT(func_tuple))->protransform;
-	if (!newexpr && OidIsValid(transform) && oldexpr)
-		newexpr = (Expr *) DatumGetPointer(OidFunctionCall1(transform,
-												  PointerGetDatum(oldexpr)));
+	if (!newexpr && allow_non_const && OidIsValid(func_form->protransform))
+	{
+		/*
+		 * Build a dummy FuncExpr node containing the simplified arg list.  We
+		 * use this approach to present a uniform interface to the transform
+		 * function regardless of how the function is actually being invoked.
+		 */
+		FuncExpr	fexpr;
 
-	if (!newexpr && allow_inline)
+		fexpr.xpr.type = T_FuncExpr;
+		fexpr.funcid = funcid;
+		fexpr.funcresulttype = result_type;
+		fexpr.funcretset = func_form->proretset;
+		fexpr.funcformat = COERCE_DONTCARE;
+		fexpr.funccollid = result_collid;
+		fexpr.inputcollid = input_collid;
+		fexpr.args = *args;
+		fexpr.location = -1;
+
+		newexpr = (Expr *)
+			DatumGetPointer(OidFunctionCall1(func_form->protransform,
+											 PointerGetDatum(&fexpr)));
+	}
+
+	if (!newexpr && allow_non_const)
 		newexpr = inline_function(funcid, result_type, result_collid,
 								  input_collid, *args,
 								  func_tuple, context);

src/backend/parser/parse_clause.c

@@ -2271,25 +2271,3 @@ transformFrameOffset(ParseState *pstate, int frameOptions, Node *clause)
 
 	return node;
 }
-
-/*
- * relabel_to_typmod
- *		Add a RelabelType node that changes just the typmod, and remove all
- *		now-superfluous RelabelType nodes beneath it.
- */
-Node *
-relabel_to_typmod(Node *expr, int32 typmod)
-{
-	Oid			type = exprType(expr);
-	Oid			coll = exprCollation(expr);
-
-	/*
-	 * Strip any existing RelabelType, then add one. This is to preserve the
-	 * invariant of no redundant RelabelTypes.
-	 */
-	while (IsA(expr, RelabelType))
-		expr = (Node *) ((RelabelType *) expr)->arg;
-
-	return (Node *) makeRelabelType((Expr *) expr, type, typmod, coll,
-									COERCE_DONTCARE);
-}

src/backend/utils/adt/datetime.c

@@ -24,7 +24,6 @@
 #include "funcapi.h"
 #include "miscadmin.h"
 #include "nodes/nodeFuncs.h"
-#include "parser/parse_clause.h"
 #include "utils/builtins.h"
 #include "utils/date.h"
 #include "utils/datetime.h"
@@ -4153,32 +4152,34 @@ CheckDateTokenTables(void)
 }
 
 /*
- * Helper for temporal protransform functions.  Types time, timetz, timestamp
- * and timestamptz each have a range of allowed precisions.  An unspecified
- * precision is rigorously equivalent to the highest specifiable precision.
+ * Common code for temporal protransform functions.  Types time, timetz,
+ * timestamp and timestamptz each have a range of allowed precisions.  An
+ * unspecified precision is rigorously equivalent to the highest specifiable
+ * precision.
+ *
+ * Note: timestamp_scale throws an error when the typmod is out of range, but
+ * we can't get there from a cast: our typmodin will have caught it already.
  */
 Node *
 TemporalTransform(int32 max_precis, Node *node)
 {
 	FuncExpr   *expr = (FuncExpr *) node;
-	Node	   *typmod;
 	Node	   *ret = NULL;
+	Node	   *typmod;
 
-	if (!IsA(expr, FuncExpr))
-		return ret;
+	Assert(IsA(expr, FuncExpr));
+	Assert(list_length(expr->args) >= 2);
 
-	Assert(list_length(expr->args) == 2);
-	typmod = lsecond(expr->args);
+	typmod = (Node *) lsecond(expr->args);
 
-	if (IsA(typmod, Const))
+	if (IsA(typmod, Const) && !((Const *) typmod)->constisnull)
 	{
-		Node	   *source = linitial(expr->args);
+		Node	   *source = (Node *) linitial(expr->args);
 		int32		old_precis = exprTypmod(source);
 		int32		new_precis = DatumGetInt32(((Const *) typmod)->constvalue);
 
-		if (new_precis == -1 ||
-			new_precis == max_precis ||
-			(old_precis != -1 && new_precis >= old_precis))
+		if (new_precis < 0 || new_precis == max_precis ||
+			(old_precis >= 0 && new_precis >= old_precis))
 			ret = relabel_to_typmod(source, new_precis);
 	}
 

src/backend/utils/adt/numeric.c

@@ -31,7 +31,6 @@
 #include "libpq/pqformat.h"
 #include "miscadmin.h"
 #include "nodes/nodeFuncs.h"
-#include "parser/parse_clause.h"
 #include "utils/array.h"
 #include "utils/builtins.h"
 #include "utils/int8.h"
@@ -717,7 +716,7 @@ numeric_send(PG_FUNCTION_ARGS)
 /*
  * numeric_transform() -
  *
- * Flatten calls to our length coercion function that solely represent
+ * Flatten calls to numeric's length coercion function that solely represent
  * increases in allowable precision.  Scale changes mutate every datum, so
  * they are unoptimizable.  Some values, e.g. 1E-1001, can only fit into an
  * unconstrained numeric, so a change from an unconstrained numeric to any
@@ -727,18 +726,17 @@ Datum
 numeric_transform(PG_FUNCTION_ARGS)
 {
 	FuncExpr   *expr = (FuncExpr *) PG_GETARG_POINTER(0);
-	Node	   *typmod;
 	Node	   *ret = NULL;
+	Node	   *typmod;
 
-	if (!IsA(expr, FuncExpr))
-		PG_RETURN_POINTER(ret);
+	Assert(IsA(expr, FuncExpr));
+	Assert(list_length(expr->args) >= 2);
 
-	Assert(list_length(expr->args) == 2);
-	typmod = lsecond(expr->args);
+	typmod = (Node *) lsecond(expr->args);
 
-	if (IsA(typmod, Const))
+	if (IsA(typmod, Const) && !((Const *) typmod)->constisnull)
 	{
-		Node	   *source = linitial(expr->args);
+		Node	   *source = (Node *) linitial(expr->args);
 		int32		old_typmod = exprTypmod(source);
 		int32		new_typmod = DatumGetInt32(((Const *) typmod)->constvalue);
 		int32		old_scale = (old_typmod - VARHDRSZ) & 0xffff;
@@ -748,11 +746,12 @@ numeric_transform(PG_FUNCTION_ARGS)
 
 		/*
 		 * If new_typmod < VARHDRSZ, the destination is unconstrained; that's
-		 * always OK.  If old_typmod >= VARHDRSZ, the source is constrained.
-		 * and we're OK if the scale is unchanged and the precison is not
+		 * always OK.  If old_typmod >= VARHDRSZ, the source is constrained,
+		 * and we're OK if the scale is unchanged and the precision is not
 		 * decreasing.  See further notes in function header comment.
 		 */
-		if (new_typmod < VARHDRSZ || (old_typmod >= VARHDRSZ &&
+		if (new_typmod < (int32) VARHDRSZ ||
+			(old_typmod >= (int32) VARHDRSZ &&
 			 new_scale == old_scale && new_precision >= old_precision))
 			ret = relabel_to_typmod(source, new_typmod);
 	}

src/backend/utils/adt/timestamp.c

@@ -28,7 +28,6 @@
 #include "libpq/pqformat.h"
 #include "miscadmin.h"
 #include "nodes/nodeFuncs.h"
-#include "parser/parse_clause.h"
 #include "parser/scansup.h"
 #include "utils/array.h"
 #include "utils/builtins.h"
@@ -316,10 +315,6 @@ timestamptypmodout(PG_FUNCTION_ARGS)
 Datum
 timestamp_transform(PG_FUNCTION_ARGS)
 {
-	/*
-	 * timestamp_scale throws an error when the typmod is out of range, but we
-	 * can't get there from a cast: our typmodin will have caught it already.
-	 */
 	PG_RETURN_POINTER(TemporalTransform(MAX_TIMESTAMP_PRECISION,
 										(Node *) PG_GETARG_POINTER(0)));
 }
@@ -937,18 +932,17 @@ Datum
 interval_transform(PG_FUNCTION_ARGS)
 {
 	FuncExpr   *expr = (FuncExpr *) PG_GETARG_POINTER(0);
-	Node	   *typmod;
 	Node	   *ret = NULL;
+	Node	   *typmod;
 
-	if (!IsA(expr, FuncExpr))
-		PG_RETURN_POINTER(ret);
+	Assert(IsA(expr, FuncExpr));
+	Assert(list_length(expr->args) >= 2);
 
-	Assert(list_length(expr->args) == 2);
-	typmod = lsecond(expr->args);
+	typmod = (Node *) lsecond(expr->args);
 
-	if (IsA(typmod, Const))
+	if (IsA(typmod, Const) && !((Const *) typmod)->constisnull)
 	{
-		Node	   *source = linitial(expr->args);
+		Node	   *source = (Node *) linitial(expr->args);
 		int32		old_typmod = exprTypmod(source);
 		int32		new_typmod = DatumGetInt32(((Const *) typmod)->constvalue);
 		int			old_range;
@@ -958,7 +952,7 @@ interval_transform(PG_FUNCTION_ARGS)
 		int			new_range_fls;
 		int			old_range_fls;
 
-		if (old_typmod == -1)
+		if (old_typmod < 0)
 		{
 			old_range = INTERVAL_FULL_RANGE;
 			old_precis = INTERVAL_FULL_PRECISION;
@@ -978,11 +972,9 @@ interval_transform(PG_FUNCTION_ARGS)
 		 */
 		new_range_fls = fls(new_range);
 		old_range_fls = fls(old_range);
-		if (new_typmod == -1 ||
-			((new_range_fls >= SECOND ||
-			  new_range_fls >= old_range_fls) &&
-			 (old_range_fls < SECOND ||
-			  new_precis >= MAX_INTERVAL_PRECISION ||
+		if (new_typmod < 0 ||
+			((new_range_fls >= SECOND || new_range_fls >= old_range_fls) &&
+			 (old_range_fls < SECOND || new_precis >= MAX_INTERVAL_PRECISION ||
 			  new_precis >= old_precis)))
 			ret = relabel_to_typmod(source, new_typmod);
 	}
@@ -1068,7 +1060,7 @@ AdjustIntervalForTypmod(Interval *interval, int32 typmod)
 		 * can't do it consistently.  (We cannot enforce a range limit on the
 		 * highest expected field, since we do not have any equivalent of
 		 * SQL's <interval leading field precision>.)  If we ever decide to
-		 * revisit this, interval_transform will likely requite adjusting.
+		 * revisit this, interval_transform will likely require adjusting.
 		 *
 		 * Note: before PG 8.4 we interpreted a limited set of fields as
 		 * actually causing a "modulo" operation on a given value, potentially

src/backend/utils/adt/varbit.c

@@ -19,7 +19,6 @@
 #include "access/htup.h"
 #include "libpq/pqformat.h"
 #include "nodes/nodeFuncs.h"
-#include "parser/parse_clause.h"
 #include "utils/array.h"
 #include "utils/varbit.h"
 
@@ -649,31 +648,31 @@ varbit_send(PG_FUNCTION_ARGS)
 
 /*
  * varbit_transform()
- * Flatten calls to our length coercion function that leave the new maximum
- * length >= the previous maximum length.  We ignore the isExplicit argument,
- * which only affects truncation.
+ * Flatten calls to varbit's length coercion function that set the new maximum
+ * length >= the previous maximum length.  We can ignore the isExplicit
+ * argument, since that only affects truncation cases.
  */
 Datum
 varbit_transform(PG_FUNCTION_ARGS)
 {
 	FuncExpr   *expr = (FuncExpr *) PG_GETARG_POINTER(0);
-	Node	   *typmod;
 	Node	   *ret = NULL;
+	Node	   *typmod;
 
-	if (!IsA(expr, FuncExpr))
-		PG_RETURN_POINTER(ret);
+	Assert(IsA(expr, FuncExpr));
+	Assert(list_length(expr->args) >= 2);
 
-	Assert(list_length(expr->args) == 3);
-	typmod = lsecond(expr->args);
+	typmod = (Node *) lsecond(expr->args);
 
-	if (IsA(typmod, Const))
+	if (IsA(typmod, Const) && !((Const *) typmod)->constisnull)
 	{
-		Node	   *source = linitial(expr->args);
+		Node	   *source = (Node *) linitial(expr->args);
 		int32		new_typmod = DatumGetInt32(((Const *) typmod)->constvalue);
 		int32		old_max = exprTypmod(source);
 		int32		new_max = new_typmod;
 
-		if (new_max <= 0 || (old_max >= 0 && old_max <= new_max))
+		/* Note: varbit() treats typmod 0 as invalid, so we do too */
+		if (new_max <= 0 || (old_max > 0 && old_max <= new_max))
 			ret = relabel_to_typmod(source, new_typmod);
 	}
 

src/backend/utils/adt/varchar.c

@@ -19,7 +19,6 @@
 #include "access/tuptoaster.h"
 #include "libpq/pqformat.h"
 #include "nodes/nodeFuncs.h"
-#include "parser/parse_clause.h"
 #include "utils/array.h"
 #include "utils/builtins.h"
 #include "mb/pg_wchar.h"
@@ -551,31 +550,32 @@ varcharsend(PG_FUNCTION_ARGS)
 
 
 /*
- * Flatten calls to our length coercion function that leave the new maximum
- * length >= the previous maximum length.  We ignore the isExplicit argument,
- * which only affects truncation.
+ * varchar_transform()
+ * Flatten calls to varchar's length coercion function that set the new maximum
+ * length >= the previous maximum length.  We can ignore the isExplicit
+ * argument, since that only affects truncation cases.
  */
 Datum
 varchar_transform(PG_FUNCTION_ARGS)
 {
 	FuncExpr   *expr = (FuncExpr *) PG_GETARG_POINTER(0);
-	Node	   *typmod;
 	Node	   *ret = NULL;
+	Node	   *typmod;
 
-	if (!IsA(expr, FuncExpr))
-		PG_RETURN_POINTER(ret);
+	Assert(IsA(expr, FuncExpr));
+	Assert(list_length(expr->args) >= 2);
 
-	Assert(list_length(expr->args) == 3);
-	typmod = lsecond(expr->args);
+	typmod = (Node *) lsecond(expr->args);
 
-	if (IsA(typmod, Const))
+	if (IsA(typmod, Const) && !((Const *) typmod)->constisnull)
 	{
-		Node	   *source = linitial(expr->args);
+		Node	   *source = (Node *) linitial(expr->args);
+		int32		old_typmod = exprTypmod(source);
 		int32		new_typmod = DatumGetInt32(((Const *) typmod)->constvalue);
-		int32		old_max = exprTypmod(source) - VARHDRSZ;
+		int32		old_max = old_typmod - VARHDRSZ;
 		int32		new_max = new_typmod - VARHDRSZ;
 
-		if (new_max < 0 || (old_max >= 0 && old_max <= new_max))
+		if (new_typmod < 0 || (old_typmod >= 0 && old_max <= new_max))
 			ret = relabel_to_typmod(source, new_typmod);
 	}