function mgm_coupon_used_users($coupon_id = NULL){



    global $wpdb;

    @ini_set( 'memory_limit', apply_filters( 'admin_memory_limit', '2048M' ) );

    $users = array();



    if(!is_null($coupon_id)){



        $start = 0;

        $limit = 1000;



        $qry  = "SELECT count(DISTINCT {$wpdb->users}.ID) ";

        $qry .= "FROM {$wpdb->users} ";

        $qry .= "INNER JOIN {$wpdb->usermeta} ON ( {$wpdb->users}.ID = {$wpdb->usermeta}.user_id ) ";

        $qry .= "INNER JOIN {$wpdb->usermeta} AS mt1 ON ( {$wpdb->users}.ID = mt1.user_id ) ";

        $qry .= "INNER JOIN {$wpdb->usermeta} AS mt2 ON ( {$wpdb->users}.ID = mt2.user_id ) ";

        $qry .= "WHERE 1 = 1 AND (({$wpdb->usermeta}.meta_key =  '_mgm_user_register_coupon' AND CAST( {$wpdb->usermeta}.meta_value AS UNSIGNED ) =  '{$coupon_id}') ";

        $qry .= "OR (mt1.meta_key =  '_mgm_user_upgrade_coupon' AND CAST( mt1.meta_value AS UNSIGNED ) =  '{$coupon_id}') ";

        $qry .= "OR (mt2.meta_key =  '_mgm_user_extend_coupon' AND CAST( mt2.meta_value AS UNSIGNED ) =  '{$coupon_id}')) ";



        $count  = $wpdb->get_var($qry);



        if($count) {

            // read again

            for( $i = $start; $i < $count; $i = $i + $limit ) {

                $users = array_merge($users, (array)mgm_patch_partial_coupon_used_users($i, $limit, $coupon_id));

                //a small delay of 0.01 second

                usleep(10000);

            }

        }

    }



    return $users;

}

/**

 * patch partial coupon used users

 */

function mgm_patch_partial_coupon_used_users($start, $limit,$coupon_id = NULL) {

    global $wpdb;



    $results = array();



    if(!is_null($coupon_id)){



        $qry  = "SELECT DISTINCT SQL_CALC_FOUND_ROWS {$wpdb->users}.ID, {$wpdb->users}.display_name, {$wpdb->users}.user_email, {$wpdb->users}.user_login, ";

        $qry .= "(SELECT t.meta_value FROM {$wpdb->usermeta} t WHERE t.user_id = {$wpdb->users}.ID AND t.meta_key =  'mgm_member_options') mgm_member_options ";

        $qry .= "FROM {$wpdb->users} ";

        $qry .= "INNER JOIN {$wpdb->usermeta} ON ( {$wpdb->users}.ID = {$wpdb->usermeta}.user_id ) ";

        $qry .= "INNER JOIN {$wpdb->usermeta} AS mt1 ON ( {$wpdb->users}.ID = mt1.user_id ) ";

        $qry .= "INNER JOIN {$wpdb->usermeta} AS mt2 ON ( {$wpdb->users}.ID = mt2.user_id ) ";

        $qry .= "WHERE 1 = 1 AND (({$wpdb->usermeta}.meta_key =  '_mgm_user_register_coupon' AND CAST( {$wpdb->usermeta}.meta_value AS UNSIGNED ) =  '{$coupon_id}') ";

        $qry .= "OR (mt1.meta_key =  '_mgm_user_upgrade_coupon' AND CAST( mt1.meta_value AS UNSIGNED ) =  '{$coupon_id}') ";

        $qry .= "OR (mt2.meta_key =  '_mgm_user_extend_coupon' AND CAST( mt2.meta_value AS UNSIGNED ) =  '{$coupon_id}')) ";

        $qry .= "ORDER BY user_login ASC LIMIT ". $start.",".$limit;



        $results =  $wpdb->get_results($qry);



        return $results;

    }

    return $results;

}