Cast functions can be immutable or stable.

doc/src/sgml/ref/create_cast.sgml

-<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/create_cast.sgml,v 1.3 2002/09/01 02:37:02 tgl Exp $ -->
+<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/create_cast.sgml,v 1.4 2002/09/15 13:04:16 petere Exp $ -->
 
 <refentry id="SQL-CREATECAST">
  <refmeta>
@@ -117,7 +117,7 @@ INSERT INTO foo(f1) VALUES(42);
        be schema-qualified.  If it is not, the function will be looked
        up in the path.  The argument type must be identical to the
        source type, the result data type must match the target type of
-       the cast.  Cast functions must be marked immutable.
+       the cast.  Cast functions must be marked immutable or stable.
       </para>
      </listitem>
     </varlistentry>

src/backend/commands/functioncmds.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/commands/functioncmds.c,v 1.19 2002/09/04 20:31:15 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/commands/functioncmds.c,v 1.20 2002/09/15 13:04:16 petere Exp $
  *
  * DESCRIPTION
  *	  These routines take the parse tree and pick out the
@@ -676,8 +676,8 @@ CreateCast(CreateCastStmt *stmt)
 			elog(ERROR, "argument of cast function must match source data type");
 		if (procstruct->prorettype != targettypeid)
 			elog(ERROR, "return data type of cast function must match target data type");
-		if (procstruct->provolatile != PROVOLATILE_IMMUTABLE)
-			elog(ERROR, "cast function must be immutable");
+		if (procstruct->provolatile == PROVOLATILE_VOLATILE)
+			elog(ERROR, "cast function must not be volatile");
 		if (procstruct->proisagg)
 			elog(ERROR, "cast function must not be an aggregate function");
 		if (procstruct->proretset)