Think\Db\Query::builder PHP Method

    protected function builder()
    {
        static $builder = [];
        $driver = $this->builder;
        if (!isset($builder[$driver])) {
            $class = false !== strpos($driver, '\\') ? $driver : '\\think\\db\\builder\\' . ucfirst($driver);
            $builder[$driver] = new $class($this->connection);
        }
        // 设置当前查询对象
        $builder[$driver]->setQuery($this);
        return $builder[$driver];
    }