自己写的高性能获取随机帖子代码，分享给大家，不足之处请高手指点！为什么不直接用MySQL语句，据我测试发现兼容性不好，不知道是否我MySQL配置问题。如果您有更好的方法请多指！
代码如下：

function get_rand_thread($num = 20, $cond = array()){

    global $db,$forumlist_show;

    $tablepre = $db->tablepre;

    $fids = arrlist_values($forumlist_show, 'fid');

    $cond['fid'] = $fids;

    $condsql = db_cond_to_sqladd($cond);

    $maxmincount = db_sql_find_one("SELECT MAX(tid), MIN(tid), COUNT(*) FROM {$tablepre}thread {$condsql}");

    list($max, $min, $count) = array_values($maxmincount);

    if($count <= $num){

        //符合条件的数据比指定的数据还少，直接全部读取吧，打散一下

        $sql = "SELECT * FROM {$tablepre}thread {$condsql}";

        $threads = db_sql_find($sql);

        shuffle($threads);

    } else {

        //随机读取帖子，万一存在不符合条件的帖子，数量就少于$num个了，那继续读取，直到数量足够为止

        $threads = array();

        do {

            $rndtids = array();

            //生成随机tid，防止重复

            for($i=$num;$i>0;$i--){

                do {

                    $rand = rand($min, $max);

                } while(in_array($rand, $rndtids));

                $rndtids[] = $rand;

            }

            $cond['tid'] = $rndtids;

            $condsql = db_cond_to_sqladd($cond);

            $sql = "SELECT * FROM {$tablepre}thread {$condsql}";

            $rndthreads = db_sql_find($sql);

            $threads += $rndthreads;

        } while(count($threads)<$num);

    }

    //数据格式化一下

    if($threads) foreach ($threads as &$thread) thread_format($thread);

    return $threads;

}