current_timestamp after queries

Hi all,
I'm trying the following:
BEGIN;
select current_timestamp into mytable;
.
some queries
.
insert current timestamp into mytable;
COMMIT;
When I call this with the \i <filename> command, all is working fine, but the two current_timestamp entries are the same, there is no difference between them but there should. So I've tried:
BEGIN;
select current_timestamp into mytable;
.
some queries
.
COMMIT;
BEGIN;
insert current_timestamp into mytable;
COMMIT;
and now the entries are different.
I think that the accuracy is not good enough because I've started two BEGIN statements and some time is elapsing between them. Am I right?
Or does anybody know a better solution to store the elapsed time after some queries without writing some code in C or JAVA?

Thanks in advance
Guido Staub