Memory Error in PHP Using SQL Query -

i have php page has search box , searches media large database. have made if put first 3 characters (e.g. v75) v75 tapes shown (v75000, v75001, etc.). when search v7 gives me error:

fatal error: allowed memory size of 268435456 bytes exhausted (tried allocate 47 bytes) on line 68

    $query->execute(); // line 66      $result = $query->fetchall(pdo::fetch_assoc); // line 68     }catch (exception $e) // line 69     {     die('cant fetch rows.'); // line 70 

i wondering must change display v7 tapes well?
approximate number of 'v7' tapes 255000

note that, if set php memory limit higher allow many results, sending more 256 mb user search query make request terribly slow if doesn't time out.

use sort of pagination , limit queries.