Laravel Eloquent读取大量记录并遍历的方法

方法1：分块结果 chunk

function test4()
{
    $start = microtime(true);
    $num = 0;
    MapCity::chunk(10, function () use (&$num) {
        $num += 10;
    });

    $end = microtime(true);

    $this->info($num);
    $this->info($end-$start);
}

3410
1.0684580802917

查看SQL日志，发现chunk方法居然是分页读取的，效率其实很低的，从时间上就能看出来。
2020-01-10 03:16:42: select * from `map_city` order by `map_city`.`id` asc limit 10 offset 0
2020-01-10 03:16:42: select * from `map_city` order by `map_city`.`id` asc limit 10 offset 10
2020-01-10 03:16:42: select * from `map_city` order by `map_city`.`id` asc limit 10 offset 20

…

方法2：游标

function test5()
{
    $start = microtime(true);
    $num = 0;
    foreach (MapCity::cursor() as $v) {
        $num++;
    }

    $end = microtime(true);

    $this->info($num);
    $this->info($end-$start);
}

3408
0.032788038253784

查看SQL日志，发现cursor方法只读取一次
2020-01-10 03:16:43: select * from `map_city`

官方如是说：该游标只执行一个查询。处理大量数据时，使用 cursor 方法可以大幅度减少内存的使用量。

在便利的情况下能大量节省内存开销，很有可能使用了生成器，于是就去看一下源码。

Illuminate\Database\Connection 第344行：

public function cursor($query, $bindings = [], $useReadPdo = true)
{
    $statement = $this->run($query, $bindings, function ($query, $bindings) use ($useReadPdo) {
        if ($this->pretending()) {
            return [];
        }

        // First we will create a statement for the query. Then, we will set the fetch
        // mode and prepare the bindings for the query. Once that's done we will be
        // ready to execute the query against the database and return the cursor.
        $statement = $this->prepared($this->getPdoForSelect($useReadPdo)
                          ->prepare($query));

        $this->bindValues(
            $statement, $this->prepareBindings($bindings)
        );

        // Next, we'll execute the query against the database and return the statement
        // so we can return the cursor. The cursor will use a PHP generator to give
        // back one row at a time without using a bunch of memory to render them.
        $statement->execute();

        return $statement;
    });

    while ($record = $statement->fetch()) {
        yield $record;
    }
}

在解析PDO结果集的时候不是一次性转换，而是使用生成器的方式逐条获取，的确在处理大量集合的时候使用生成器的方式
来遍历能大幅度减小内存开销。