PHP产量与PDO获取？

Question

Yesterday, I've learned that PHP has a yield() method. I was unure about its usefulness in PHP.

A colleague said it could help for SQL statements returning many rows causing potential memory issues. I believe he was refering to fetchAll() . But, instead of using fetchAll() , one can also use fetch() and process rows one-by-one. So, yield() is not key to solving the issue he is referring to.

Am I missing something about yield() vs fetch() here? Are there more benefits to using yield() and generators?

PS: It's true that it's easier to write clean, readable and maitainable code in large applications with yield() than with with fetch() .

Answer 1

So, yield() is not key to solving the issue he is referring to.

Exactly.

But it can let you to disguise while() / fetch() sequence as a foreach() call if you prefer to, without memory overhead as well.

However, PDO is not a very good example, because PDOStatement already implements a traversable interface and thus can be iterated over using foreach() :

$stmt = $pdo->query('SELECT name FROM users');
foreach ($stmt as $row)
{
    var_export($row);
}

So let's take mysqli for the example API that can only stream results one by one.
Edit . Actually, since 5.4.0 mysqli supports Traversable as well, so there is no point in using yield with mysqli_result either. But meh, let's keep it for the demonstration purpose.

Let's create a generator like this

function mysqli_gen (mysqli_result $res)
{
    while($row = mysqli_fetch_assoc($res))
    {
        yield $row;
    }
}

and now you can get rows using foreach without an overhead:

$res = $mysqli->query("SELECT * FROM users");
foreach (mysqli_gen($res) as $row)
{
    var_export($row);
}