For review: dbsize patch

The attached patch is an update of the dbsize integration patch
discussed last week. This version includes the following functions:

pg_relation_size(text) - Get relation size by name/schema.name
pg_relation_size(oid) - Get relation size by OID
pg_tablespace_size(name) - Get tablespace size by name
pg_tablespace_size(oid) - Get tablespace size by OID
pg_database_size(name) - Get database size by name
pg_database_size(oid) - Get database size by OID
pg_size_pretty(int8) - Pretty print (and round) the byte size
specified (eg, 123456 = 121KB)

The only remaining function that last week's brief discussion indicated
was required is a replacement for total_relation_size() (or
pg_table_size() as it might now be called). I didn't realise until a few
minutes ago that this function (which is actually broken because it
doesn't handle schemas) was only committed a couple of months ago (v1.5,
http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dbsize/dbsize.s
ql.in) and has therefore never been in a release version.

So should we include this new feature, and if so, how is it best added -
rewrite in C, or one long line in pg_proc?

Regards, Dave