Add notion of a "transform function" that can simplify function calls.

Initially, we use this only to eliminate calls to the varchar()
function in cases where the length is not being reduced and, therefore,
the function call is equivalent to a RelabelType operation.  The most
significant effect of this is that we can avoid a table rewrite when
changing a varchar(X) column to a varchar(Y) column, where Y > X.

Noah Misch, reviewed by me and Alexey Klyukin

1 files changed, 34 insertions, 0 deletions

diff --git a/src/backend/utils/adt/varchar.c b/src/backend/utils/adt/varchar.c
index 1c0ef921a7..efa483d638 100644
--- a/src/backend/utils/adt/varchar.c
+++ b/src/backend/utils/adt/varchar.c
@@ -18,6 +18,8 @@
 #include "access/hash.h"
 #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"
@@ -549,6 +551,38 @@ 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.
+ */
+Datum
+varchar_transform(PG_FUNCTION_ARGS)
+{
+	FuncExpr   *expr = (FuncExpr *) PG_GETARG_POINTER(0);
+	Node	   *typmod;
+	Node	   *ret = NULL;
+
+	if (!IsA(expr, FuncExpr))
+		PG_RETURN_POINTER(ret);
+
+	Assert(list_length(expr->args) == 3);
+	typmod = lsecond(expr->args);
+
+	if (IsA(typmod, Const))
+	{
+		Node	   *source = linitial(expr->args);
+		int32		new_typmod = DatumGetInt32(((Const *) typmod)->constvalue);
+		int32		old_max = exprTypmod(source) - VARHDRSZ;
+		int32		new_max = new_typmod - VARHDRSZ;
+
+		if (new_max < 0 || (old_max >= 0 && old_max <= new_max))
+			ret = relabel_to_typmod(source, new_typmod);
+	}
+
+	PG_RETURN_POINTER(ret);
+}
+
+/*
  * Converts a VARCHAR type to the specified size.
  *
  * maxlen is the typmod, ie, declared length plus VARHDRSZ bytes.