pg_database_size() and actual disk space usage

Hi folks,

I'm analyizing why our /var/lib/postgresql partition got a quick bump in
usage (from 32GB to 41GB) few days ago, even if, according to data from
monitoring software, database size kept increasing with no bump at all.

So I'm using "SELECT pg_database_size('mydb')" but I can't find much
documentation about what that "size" actually refers to. What I see is
that both the master and the replicas shows the same number (32GB), but
for both of those I see differences in the actual file system usage:

master:
41G     /var/lib/postgresql/9.1/main/base

replica:
37G     /var/lib/postgresql/9.1/main/base/

Does pg_database_size() return size occupied by indexes too? If not, the
difference could be due to index space I guess.

Thanks!

--
Alexander Fortin
http://about.me/alexanderfortin/