PHP:查询结果如何存储在 mysqli_result 中

问题描述

当我查询数据库并在mysqli_result中检索结果时，内存使用量极小.但是，当我将查询结果中的所有行提取到关联数组中时，内存使用率变得非常高.

When I made a query to the database and retrieve the results in mysqli_result, the memory usage is extremely small. However, when I fetch all the rows in the query results in to an associative array, the memory usage becomes extremely high.

<?php
    require_once("../config.php"); //db connection config
    $db = new mysqli(DB_HOST,DB_USER,DB_PASSWORD,DB_DBASE);

    $query ="select * from table_name";
    if($r = $db->query($query)){
    echo "MEMORY USAGE before :  ". memory_get_usage()."<br><br>";
    $rows = array();
    while($row = $r->fetch_assoc()){

        $rows[]= $row;
    }
    echo "MEMORY USAGE after :  ". memory_get_usage()."<br><br>";


    //before: 660880
    //after:  114655768
    // # of records: around 30 thousands
?>

对我来说，存储这么多结果非常消耗内存是有道理的，但我只是想知道 mysqli_result 怎么这么小.不可能是每次调用fetch_assoc的时候都向dbase查询结果.那么结果存储在内存中的什么地方.

It makes sense to me that storing this many results is very memory consuming, but I'm just wondering how come mysqli_result is so small. It can't be that the results are queried to the dbase every time fetch_assoc is called. So then where are the results stored in the memory.

推荐答案

获取结果和存储指向资源的指针之间存在巨大差异.

There is a HUGE difference between fetching results and storing a pointer to a resource.

如果你在第一次调用 memory_get_usage(); 之前 echo $r;，你会意识到它只是一个指针.这是指向结果集的指针.在您获取结果之前，结果集实际上不会存储到内存中.

If you echo $r; before your first call to memory_get_usage();, you will realize it is just a pointer. This is the pointer to your result set. Until you fetch your results, the result set will not actually be stored into memory.

我建议您运行 fetchAll() 来执行您要执行的操作.这将导致 1 个方法以更好的性能访问您的所有结果，因为它是在 mysqli 扩展(C 库)而不是 PHP 中的循环上典当的.

I would suggest that you run fetchAll() for what you are trying to do. This will then result in 1 method accessing all your results with better performance since it's pawned off on the mysqli extension (C Library) rather than a loop in PHP.

您还可以使用免费结果功能在完成后从内存中清除结果.如果您熟悉，这就像在 Java 中关闭游标一样.

You can also use the free results function to clear your results from memory when you are done with them. This is like closing a cursor in Java if you are familiar.

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