XLOG's implementation details

Hi all,

I am new to Postgres and am interested in XLOG's implementation
details. I browsed around the source code, and saw that the particular
function that appends XLOG entries is called from multiple places
(heap, indexes, transaction subsystem, and so forth). Seeing this, I
became particularly interested in the following:

How strong is the coupling between XLOG entries and database pages?
That is, assume that I am inserting one row to a database table. Is
there anyway for me to construct the related XLOG entry before calling
the heap insert function? Or, are XLOG files conceptually binary diffs
for database pages, moving forward in time?

Another question that I had relates to XLOG files and tables. If I
have two databases and one is set up as a slave to the other, is it
possible for the slave to have fewer tables than the master? If it is
possible, is that because tables don't share pages?

Again I'm new to Postgres, and am curious about how stuff works
underneath the covers. I figured asking this list would be the
simplest way to figure things out.

Thanks,

Ozgun.