psql (9.2devel)
Type "help" for help.

postgres=# select pg_expo

postgres=# select pg_export_snapshot();
 pg_export_snapshot 
--------------------
 000002C8-1
(1 row)

postgres=# begin transaction;
BEGIN
postgres=# select pg_export_snapshot();
 pg_export_snapshot 
--------------------
 000002C9-1
(1 row)

postgres=# end;
COMMIT
postgres=# select pg_export_snapshot();
 pg_export_snapshot 
--------------------
 000002CA-1
(1 row)

postgres=# \q