levels.go
func (s *levelsController) appendIterators(
	iters []y.Iterator, opt *IteratorOptions) []y.Iterator {
	// Just like with get, it's important we iterate the levels from 0 on upward, to avoid missing
	// data when there's a compaction.
	for _, level := range s.levels {
		iters = level.appendIterators(iters, opt)
	}
	return iters
}
func (s *levelsController) appendIterators(
	iters []y.Iterator, opt *IteratorOptions) []y.Iterator {
	// Just like with get, it's important we iterate the levels from 0 on upward, to avoid missing
	// data when there's a compaction.
	for _, level := range s.levels {
		iters = level.appendIterators(iters, opt)
	}
	return iters
}
// appendIterators appends iterators to an array of iterators, for merging.
// Note: This obtains references for the table handlers. Remember to close these iterators.