laravel cursor的底层实现

foreach (BillInfo::where('id', '>', 0)->orderBy('id', 'desc')->limit(10000)->cursor() as $value) {
    print_r($value);
}

Illuminate\Database\Query\Builder.php;

/**
 * Get a generator for the given query.
 *
 * @return \Generator
 */
public function cursor()
{
    if (is_null($this->columns)) {
        $this->columns = ['*'];
    }

    return $this->connection->cursor(
        $this->toSql(), $this->getBindings(), ! $this->useWritePdo
    );
}

Illuminate\Database\Connection.php;

/**
 * Run a select statement against the database and returns a generator.
 *
 * @param  string  $query
 * @param  array  $bindings
 * @param  bool  $useReadPdo
 * @return \Generator
 */
public function cursor($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;
    }
}