After creating a table with a 'serial' type id field I noticed this when describing it:
sql>CREATE TABLE "schtest"."testt" ("id" serial, ...
sql>\d testt
CREATE TABLE "schtest"."testt" (
"id" int NOT NULL DEFAULT next value for "schtest"."seq_4291",
...

The following page:
http://monetdb.cwi.nl/projects/monetdb/SQL/Documentation/Data-Types.html

Says this about the serial data type:
"special 64 bit integer (sequence generator)"

and this about the 'int' data type:
"32 bit integer"

This looks like it created a table with a 32-bit integer sequence rather than a 64-bit integer sequence.

If you use the auto_increment method you can set the type explicitly to be bigint, but should "serial" generate a bigint column also?

73,
Matthew W. Jones (KI4ZIB)
http://matburt.net