几千万的数据用 mysql 查询就很慢

<?php
session_start();
error_reporting(0);
header("Content-type: text/html; charset=utf-8");
require "config.php";
@date_default_timezone_set(PRC);
set_time_limit(0);
@ob_end_clean();
ob_implicit_flush(true);
switch($_GET['act']){
case "database":
if(empty($_SESSION['member'])){
exit('var database=new Array("login");');
}
$connect_db = mysql_connect($dbnhost, $dbnuser, $dbnpass);
$select_db = mysql_select_db($dbname, $connect_db);
mysql_query("SET NAMES 'UTF8'");
mysql_query("SET CHARACTER SET UTF8");
mysql_query("SET CHARACTER_SET_RESULTS=UTF8");
$rs = mysql_query("SHOW TABLES FROM $dbname");
$tables = array();
while ($row = mysql_fetch_row($rs)) {
$tables[] = $row[0];
}
mysql_free_result($rs);
$array_tj=count($tables);
$count=1;
$text="";
foreach($tables as $key=>$tableName){
if($key==count($tables)-1){
$dian="";
}else{
$dian=",";
}
$text=$text.'"'.$tableName.'"'.$dian;
$count++;
}
echo "var database = new Array($text);";

break;
case "select":
if(empty($_SESSION['member'])){
echo "cnrv_msg(\"请登录\");addRow(\"登录后查询\",\"登录后查询\",\"登录后查询\",\"登录后查询\");";
exit;

}
        $select_act=(int)addslashes(trim($_POST['select_act']));
        $match_act=(int)addslashes(trim($_POST['match_act']));
        $key=addslashes(trim($_POST['key']));
        $table=addslashes(trim($_POST['table']));
            if(empty($key) || $key==''){exit("请输入查询内容");}
            if(strlen($key)<4){exit("key length!!!");}

                $key = str_replace("_","\_",$key);
                $key = str_replace("%","\%",$key);
                    switch($match_act){
                        case 2:$key = '=\''.$key.'\'';break;
                        case 1:$key = ' like \''.$key.'%\'';break;
                        default:exit("fuck you!");
                    }
                    switch($select_act){//查询方式
                        case 1:$limits="username".$key;break;
                        case 2:$limits="email".$key;break;
                        case 3:$limits="username".$key."or email".$key;break;
                        default:exit("fuck you!");
                    }
                        $connect_db = mysql_connect($dbnhost, $dbnuser, $dbnpass);
                        $select_db = mysql_select_db($dbname, $connect_db);
                        mysql_query("SET NAMES 'UTF8'");
                        mysql_query("SET CHARACTER SET UTF8");
                        mysql_query("SET CHARACTER_SET_RESULTS=UTF8");
                    $sql="select $Field  from `$table` where $limits LIMIT 20";
                    require "database.php";
                        $databasename=database($table);
                        if($result=mysql_query($sql)){
                            while($rows=mysql_fetch_assoc($result)){
                                    $username= mysql_real_escape_string($rows['username']);
                                    $email= mysql_real_escape_string($rows['email']);
                                    $password= mysql_real_escape_string($rows['password']);
                                    echo "addRow(\"$username\",\"$email\",\"$password\",\"$databasename\");";
                            }// end while
                        }




    break;
    default:print_r("fuck you!");
}

索引FULLTEXT建立了。还是慢。几千万就用了30~配置双核2g内存ssd