Logging access to data in database table

Hello,

I need to log access to certain data in database in some log (I prefer
to have that both in syslog and table in database), and I find it easy
to write to syslog, but I can't solve the problem of writing this to
database table.

If this protected data is read only using postgres function , and if in
the same function I add something like "insert into log_table (blah blah
blah)", somebody could simply do
begin;
select * from access_function(); /* assuming access_function is function
for accessing sensitive data */
rollback;

and no info about access would be written in log_table.

Is there some way to enforce insert within function to be always
performed (I checked and commit can't be called within functions), or is
there maybe some completely different clever way to solve this problem?

Thanks in advance,
Ivan