I read somewhere that the following query gives a quick estimate of the # of rows in a table regardless of the table's size (which would matter in a simple SELECT count(*)?):
SELECT (CASE WHEN reltuples > 0 THEN pg_relation_size('mytable')/(8192*relpages/reltuples) ELSE 0 END)::bigint AS estimated_row_count FROM pg_class WHERE oid = 'mytable'::regclass;
If relpages & reltuples are recorded accurately each time VACUUM is run, wouldn't it be the same to just grab directly the value of reltuples like:
SELECT reltuples FROM pg_class WHERE oid='mytable'::regclass;
In the same manner, are pg_relation_size('mytable') and 8192*relpages the same?
I run both assumptions against a freshly VACUUMed table and they seem correct.
