Possible to keep max number of records in postgresql?
You can define a trigger to maintain your desired row number:

CREATE OR REPLACE FUNCTION trf_keep_row_number_steady()
RETURNS TRIGGER AS
$body$
BEGIN
    -- delete only where are too many rows
    IF (SELECT count(id) FROM log_table) &gt; rownum_limit
    THEN 
        -- I assume here that id is an auto-incremented value in log_table
        DELETE FROM log_table
        WHERE id = (SELECT min(id) FROM log_table);
    END IF;
END;
$body$
LANGUAGE plpgsql;

CREATE TRIGGER tr_keep_row_number_steady 
AFTER INSERT ON log_table
FOR EACH ROW EXECUTE PROCEDURE trf_keep_row_number_steady();


This is probably not the best performing option, but once you reach the limit, it will never be exceeded.  If there is space for fluctuation, then you can check the row number periodically and delete excess rows from the beginning.

EDIT:
If you have really large logs (say a million per month) than partitioning can be the easiest solution.  You can then simply drop the unnecessary tables (say where max(timestamp) &lt; CURRENT_DATE - 1 year).  You can use your timestamp (or a derived date) as condition for range partitioning.

But be careful before discarding old logs.  Are you sure you will never need those?