Database result set question

Just out of curiosity, can anyone explain the mechanics behind accessing a database query result set. ie $result_id = pg_execute( 'select * from my_table' ); when I call loop through pg_fetch_row( $result_id, $rom_num ) or pg_fetch_object( $result_id, $rom_num ) has php read the entire result set into memory( say the result set has 10,000 records ), cached a fixed number of records in memory or does it simply have a record_set_pointer of some sort that tells the DB where to fetch the next asked for record from. This is probably a DB engine setting but I am curious what the process is. Thanks, Morgan