postgres=# WITH slru(relfilenode, path) AS (VALUES (0, 'pg_xact'),
(1, 'pg_multixact/offsets'),
(2, 'pg_multixact/members'),
(3, 'pg_subtrans'),
(4, 'pg_serial'),
(5, 'pg_commit_ts'),
(6, 'pg_notify'))
SELECT path, pg_size_pretty(COUNT(*) * 8192)
FROM pg_buffercache NATURAL JOIN slru
WHERE reldatabase = 9
GROUP BY 1
ORDER BY 1;
path | pg_size_pretty
----------------------+----------------
pg_multixact/offsets | 8192 bytes
pg_subtrans | 31 MB
pg_xact | 3560 kB
(3 rows)