具有限制和大偏移量的MySQL查询永远存在

Question

我有一个使用以下PHP脚本构建的目录，该目录使用分页来获取每页1002个结果。问题在于，您进入页面的距离越远，加载页面所需的时间就越长。例如，第1页的加载速度明显快于10,000页。

我猜我在查询中做错了什么，而不仅仅是选择应限制的1002个结果，它还循环遍历了之前的所有结果。如果有人可以发布需要修复的代码，那就太好了！

感谢您的时间和帮助！

<?php include("websites/header.html"); ?>

<center>
<?php
    /*
        Place code to connect to your DB here.
    */
    include('websites/database.php');   // include your code to connect to DB.

    $tbl_name="list";       //your table name
    // How many adjacent pages should be shown on each side?
    $adjacents = 5;

    /* 
       First get total number of rows in data table. 
       If you have a WHERE clause in your query, make sure you mirror it here.
    */
    $query = "SELECT COUNT(*) as num FROM $tbl_name";
    $total_pages = mysql_fetch_array(mysql_query($query));
    $total_pages = $total_pages[num];

    /* Setup vars for query. */
    $targetpage = "websites.php";   //your file name  (the name of this file)
    $limit = 1002;                              //how many items to show per page
    $page = $_GET['page'];
    if($page) 
        $start = ($page - 1) * $limit;          //first item to display on this page
    else
        $start = 0;                             //if no page var is given, set start to 0

    /* Get data. */
    $sql = "SELECT website FROM $tbl_name LIMIT $start, $limit";
    $result = mysql_query($sql);

    /* Setup page vars for display. */
    if ($page == 0) $page = 1;                  //if no page var is given, default to 1.
    $prev = $page - 1;                          //previous page is page - 1
    $next = $page + 1;                          //next page is page + 1
    $lastpage = ceil($total_pages/$limit);      //lastpage is = total pages / items per page, rounded up.
    $lpm1 = $lastpage - 1;                      //last page minus 1

    /* 
        Now we apply our rules and draw the pagination object. 
        We're actually saving the code to a variable in case we want to draw it more than once.
    */
    $pagination = "";
    if($lastpage > 1)
    {   
        $pagination .= "<div class=\"pagination2\">";
        //previous button
        if ($page > 1) 
            $pagination.= "<a href=\"$targetpage?page=$prev\">&lt; previous</a>";
        else
            $pagination.= "<span class=\"disabled\">&lt; previous</span>";

        //pages 
        if ($lastpage < 7 + ($adjacents * 2))   //not enough pages to bother breaking it up
        {   
            for ($counter = 1; $counter <= $lastpage; $counter++)
            {
                if ($counter == $page)
                    $pagination.= "<span class=\"current\">$counter</span>";
                else
                    $pagination.= "<a href=\"$targetpage?page=$counter\">$counter</a>";                 
            }
        }
        elseif($lastpage > 5 + ($adjacents * 2))    //enough pages to hide some
        {
            //close to beginning; only hide later pages
            if($page < 1 + ($adjacents * 2))        
            {
                for ($counter = 1; $counter < 4 + ($adjacents * 2); $counter++)
                {
                    if ($counter == $page)
                        $pagination.= "<span class=\"current\">$counter</span>";
                    else
                        $pagination.= "<a href=\"$targetpage?page=$counter\">$counter</a>";                 
                }
                $pagination.= "...";
                $pagination.= "<a href=\"$targetpage?page=$lpm1\">$lpm1</a>";
                $pagination.= "<a href=\"$targetpage?page=$lastpage\">$lastpage</a>";       
            }
            //in middle; hide some front and some back
            elseif($lastpage - ($adjacents * 2) > $page && $page > ($adjacents * 2))
            {
                $pagination.= "<a href=\"$targetpage?page=1\">1</a>";
                $pagination.= "<a href=\"$targetpage?page=2\">2</a>";
                $pagination.= "...";
                for ($counter = $page - $adjacents; $counter <= $page + $adjacents; $counter++)
                {
                    if ($counter == $page)
                        $pagination.= "<span class=\"current\">$counter</span>";
                    else
                        $pagination.= "<a href=\"$targetpage?page=$counter\">$counter</a>";                 
                }
                $pagination.= "...";
                $pagination.= "<a href=\"$targetpage?page=$lpm1\">$lpm1</a>";
                $pagination.= "<a href=\"$targetpage?page=$lastpage\">$lastpage</a>";       
            }
            //close to end; only hide early pages
            else
            {
                $pagination.= "<a href=\"$targetpage?page=1\">1</a>";
                $pagination.= "<a href=\"$targetpage?page=2\">2</a>";
                $pagination.= "...";
                for ($counter = $lastpage - (2 + ($adjacents * 2)); $counter <= $lastpage; $counter++)
                {
                    if ($counter == $page)
                        $pagination.= "<span class=\"current\">$counter</span>";
                    else
                        $pagination.= "<a href=\"$targetpage?page=$counter\">$counter</a>";                 
                }
            }
        }

        //next button
        if ($page < $counter - 1) 
            $pagination.= "<a href=\"$targetpage?page=$next\">next &gt;</a>";
        else
            $pagination.= "<span class=\"disabled\">next &gt;</span>";
        $pagination.= "</div>\n";       
    }
?>

    <?php
$i = 0;
echo '<table style="table-layout:fixed; width:1050px;"><tr>'; 
        while($row = mysql_fetch_array($result))
{
    $i ++;
    if ($i<=3)
    {
      echo '<td style="word-wrap: break-word;">
         <div><a href="http://www.mywebsite.com/check.php?site='.strtolower($row[website]).'">'.strtolower($row[website]).'</a></div>
       </td>'; 
    }

    else
    {       
      echo '</tr><tr>';
      echo '<td style="word-wrap: break-word;"><div><a href="http://www.mywebsite.com/check.php?site='.strtolower($row[website]).'">'.strtolower($row[website]).'</a></div></td>'; 
      $i = 0;
    $i++;
   }
}  
echo '</tr></table>';
    ?>

<?=$pagination?>
</center>

<?php include("websites/footer.html"); ?>

Answer 1

在大多数数据库中，带有偏移量的LIMIT极其慢（我已经找到了一些
有关MySQL的文档，并且我试图找到我之前读过的关于SQLite的一篇很好的文章）。原因是通常已实现了以下内容：

1. 进行所有常规查询计划，就好像该LIMIT子句不在那儿一样
2. 浏览结果，直到找到所需的索引
3. 开始返回结果

如果您这样做LIMIT 10000, 10，这意味着什么，它将被解释为：

1. 提取前10,000个结果并忽略它们
2. 给你接下来的10个结果

有一个微不足道的优化方法，因为您不关心它们的值，所以您至少可以对前10,000个结果使用索引，但是即使在那种情况下，数据库仍然需要遍历10,000个索引值才能给您10个结果。可能还有进一步的优化可以改善这一点，但是通常情况下，
您不想LIMIT对大值使用偏移量。

我知道，处理分页的最有效方法是跟踪最后一个索引，因此，如果第一个页面以结尾id = 5，则使您的 下一个 链接具有WHERE id > 5（LIMIT x当然带有）。

编辑：找到了SQLite的文章。我强烈建议您阅读此书，因为它解释了在SQL中执行操作的正确方法。由于SQLite的人
真的很聪明， 而其他数据库也有同样的问题，因此我认为MySQL以类似的方式实现了这一点。

经常出现的另一个错误是程序员尝试使用LIMIT和OFFSET实现滚动窗口。这里的想法是，您首先只需要记住显示中顶部条目的索引，然后运行如下查询：

SELECT title FROM tracks
WHERE singer='Madonna'
ORDER BY title
LIMIT 5 OFFSET :index

索引初始化为0。要向下滚动，只需将索引增加5并重新运行查询即可。要向上滚动，请将索引递减5并重新运行。

以上将实际工作。 问题是当索引变大时它变慢。
OFFSET在SQLite中的工作方式是使sqlite3_step（）函数忽略它看到的第一个：index断点。因此，例如，如果：index为1000，则您实际上是在读1005个条目，而忽略除最后5个之外的所有条目。最终的结果是，随着列表中的位置越来越低，滚动开始变得缓慢。