WAL log archive frequency

Hi,

Further to my question on PITR, I have now implemented it :-) However,
I was wondering about the frequency of archiving the WAL. Does
postgresql wait until the current WAL file has reached 16MB before
calling the archive_command? The reason why I ask is that last night,
after doing the base backup, I noticed that a WAL file had been written
to my backup server. This morning, I added a primary key to a table but
noticed that nothing was written to the backup server. Should it have
been?

Is there any method of forcing a WAL segment to be archived at a given
frequency irrespective of whether or not it is a full 16MB? Our
database changes so infrequently at the moment that I'm worried that
these small changes may not be backed up regularly by the PITR system.

Thanks,

Chris
--
Chris Jewell, BSc(Hons), BVSc, MRCVS
Dept of Maths and Statistics
Fylde College
Lancaster University
Lancaster
Lancs
LA1 4YF