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];
}