Add a transform function for numeric typmod coercisions.

This enables ALTER TABLE to skip table and index rebuilds when a column
is changed to an unconstrained numeric, or when the scale is unchanged
and the precision does not decrease.

Noah Misch, with a few stylistic changes and a fix for an OID
collision by me.

src/backend/utils/adt/numeric.c

@@ -30,6 +30,8 @@
 #include "catalog/pg_type.h"
 #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"
@@ -712,6 +714,52 @@ numeric_send(PG_FUNCTION_ARGS)
 }
 
 
+/*
+ * numeric_transform() -
+ *
+ * Flatten calls to our 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
+ * constrained numeric is also unoptimizable.
+ */
+Datum
+numeric_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) == 2);
+	typmod = lsecond(expr->args);
+
+	if (IsA(typmod, Const))
+	{
+		Node	   *source = linitial(expr->args);
+		int32		old_typmod = exprTypmod(source);
+		int32		new_typmod = DatumGetInt32(((Const *) typmod)->constvalue);
+		int32		old_scale = (old_typmod - VARHDRSZ) & 0xffff;
+		int32		new_scale = (new_typmod - VARHDRSZ) & 0xffff;
+		int32		old_precision = (old_typmod - VARHDRSZ) >> 16 & 0xffff;
+		int32		new_precision = (new_typmod - VARHDRSZ) >> 16 & 0xffff;
+
+		/*
+		 * If new_typmod < VARHDRSZ, the destination is unconstrained; that's
+		 * always OK.  If old_typmod >= VARHDRSZ, the source is constained.
+		 * and we're OK if the scale is unchanged and the precison is not
+		 * decreasing.  See further nodes in function header comment.
+		 */
+		if (new_typmod < VARHDRSZ || (old_typmod >= VARHDRSZ &&
+			 new_scale == old_scale && new_precision >= old_precision))
+			ret = relabel_to_typmod(source, new_typmod);
+	}
+
+	PG_RETURN_POINTER(ret);
+}
+
 /*
  * numeric() -
  *

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201201311
+#define CATALOG_VERSION_NO	201202071
 
 #endif

src/include/catalog/pg_proc.h

@@ -2142,8 +2142,10 @@ DATA(insert OID = 2917 (  numerictypmodin		PGNSP PGUID 12 1 0 0 0 f f f t f i 1
 DESCR("I/O typmod");
 DATA(insert OID = 2918 (  numerictypmodout		PGNSP PGUID 12 1 0 0 0 f f f t f i 1 0 2275 "23" _null_ _null_ _null_ _null_	numerictypmodout _null_ _null_ _null_ ));
 DESCR("I/O typmod");
-DATA(insert OID = 1703 ( numeric				PGNSP PGUID 12 1 0 0 0 f f f t f i 2 0 1700 "1700 23" _null_ _null_ _null_ _null_ numeric _null_ _null_ _null_ ));
+DATA(insert OID = 1703 ( numeric				PGNSP PGUID 12 1 0 0 3157 f f f t f i 2 0 1700 "1700 23" _null_ _null_ _null_ _null_ numeric _null_ _null_ _null_ ));
 DESCR("adjust numeric to typmod precision/scale");
+DATA(insert OID = 3157 ( numeric_transform		PGNSP PGUID 12 1 0 0 0 f f f t f i 1 0 2281 "2281" _null_ _null_ _null_ _null_ numeric_transform _null_ _null_ _null_ ));
+DESCR("transform a numeric length coercion");
 DATA(insert OID = 1704 ( numeric_abs			PGNSP PGUID 12 1 0 0 0 f f f t f i 1 0 1700 "1700" _null_ _null_ _null_ _null_ numeric_abs _null_ _null_ _null_ ));
 DATA(insert OID = 1705 ( abs					PGNSP PGUID 12 1 0 0 0 f f f t f i 1 0 1700 "1700" _null_ _null_ _null_ _null_ numeric_abs _null_ _null_ _null_ ));
 DESCR("absolute value");

src/include/utils/builtins.h

@@ -913,6 +913,7 @@ extern Datum numeric_recv(PG_FUNCTION_ARGS);
 extern Datum numeric_send(PG_FUNCTION_ARGS);
 extern Datum numerictypmodin(PG_FUNCTION_ARGS);
 extern Datum numerictypmodout(PG_FUNCTION_ARGS);
+extern Datum numeric_transform(PG_FUNCTION_ARGS);
 extern Datum numeric (PG_FUNCTION_ARGS);
 extern Datum numeric_abs(PG_FUNCTION_ARGS);
 extern Datum numeric_uminus(PG_FUNCTION_ARGS);