help !!!

["Alxander A. Kapralov" <sasha@xxxxxxxxxxx>]

function PaperList () {
       global $db;
       $str = "select gazeta.id as i,gorod.id as ig,trim(gazeta.name,' 
') as gaz_n, trim(gorod.name,' ') as gor_n from gazeta,gorod where 
gazeta.id_gorod=gorod.id order by gazeta.name asc;";
       $result = pg_Exec( $db, $str );
       if ($result = pg_Exec( $db, $str ) != false && ($num = 
pg_numrows($result)) ) {
           for ($i = 0; $i < $num; $i++) {
               $r = pg_fetch_object($result, $i);
                   ^^^^^   -- here get error message
               echo "<OPTION VALUE=$r->i> $r->gor_n / $r->gaze_n\n";
           };
       };
   };

Supplied argument is not a valid PostgreSQL result resource in ib.inc
Can any one explain that  !!!!