help !!!

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 !!!!