func (db *DB) newIterator(seq uint64, slice *util.Range, ro *opt.ReadOptions) *dbIter {
	var islice *util.Range
	if slice != nil {
		islice = &util.Range{}
		if slice.Start != nil {
			islice.Start = newIkey(slice.Start, kMaxSeq, ktSeek)
		}
		if slice.Limit != nil {
			islice.Limit = newIkey(slice.Limit, kMaxSeq, ktSeek)
		}
	}
	rawIter := db.newRawIterator(islice, ro)
	iter := &dbIter{
		db:     db,
		icmp:   db.s.icmp,
		iter:   rawIter,
		seq:    seq,
		strict: opt.GetStrict(db.s.o.Options, ro, opt.StrictReader),
		key:    make([]byte, 0),
		value:  make([]byte, 0),
	}
	atomic.AddInt32(&db.aliveIters, 1)
	runtime.SetFinalizer(iter, (*dbIter).Release)
	return iter
}

func (db *DB) newRawIterator(auxm *memDB, auxt tFiles, slice *util.Range, ro *opt.ReadOptions) iterator.Iterator {
	strict := opt.GetStrict(db.s.o.Options, ro, opt.StrictReader)
	em, fm := db.getMems()
	v := db.s.version()

	tableIts := v.getIterators(slice, ro)
	n := len(tableIts) + len(auxt) + 3
	its := make([]iterator.Iterator, 0, n)

	if auxm != nil {
		ami := auxm.NewIterator(slice)
		ami.SetReleaser(&memdbReleaser{m: auxm})
		its = append(its, ami)
	}
	for _, t := range auxt {
		its = append(its, v.s.tops.newIterator(t, slice, ro))
	}

	emi := em.NewIterator(slice)
	emi.SetReleaser(&memdbReleaser{m: em})
	its = append(its, emi)
	if fm != nil {
		fmi := fm.NewIterator(slice)
		fmi.SetReleaser(&memdbReleaser{m: fm})
		its = append(its, fmi)
	}
	its = append(its, tableIts...)
	mi := iterator.NewMergedIterator(its, db.s.icmp, strict)
	mi.SetReleaser(&versionReleaser{v: v})
	return mi
}

func (v *version) getIterators(slice *util.Range, ro *opt.ReadOptions) (its []iterator.Iterator) {
	strict := opt.GetStrict(v.s.o.Options, ro, opt.StrictReader)
	for level, tables := range v.levels {
		if level == 0 {
			// Merge all level zero files together since they may overlap.
			for _, t := range tables {
				its = append(its, v.s.tops.newIterator(t, slice, ro))
			}
		} else if len(tables) != 0 {
			its = append(its, iterator.NewIndexedIterator(tables.newIndexIterator(v.s.tops, v.s.icmp, slice, ro), strict))
		}
	}
	return
}

func (v *version) getIterators(slice *util.Range, ro *opt.ReadOptions) (its []iterator.Iterator) {
	// Merge all level zero files together since they may overlap
	for _, t := range v.tables[0] {
		it := v.s.tops.newIterator(t, slice, ro)
		its = append(its, it)
	}

	strict := opt.GetStrict(v.s.o.Options, ro, opt.StrictReader)
	for _, tables := range v.tables[1:] {
		if len(tables) == 0 {
			continue
		}

		it := iterator.NewIndexedIterator(tables.newIndexIterator(v.s.tops, v.s.icmp, slice, ro), strict)
		its = append(its, it)
	}

	return
}

func (db *DB) newRawIterator(slice *util.Range, ro *opt.ReadOptions) iterator.Iterator {
	em, fm := db.getMems()
	v := db.s.version()

	ti := v.getIterators(slice, ro)
	n := len(ti) + 2
	i := make([]iterator.Iterator, 0, n)
	emi := em.mdb.NewIterator(slice)
	emi.SetReleaser(&memdbReleaser{m: em})
	i = append(i, emi)
	if fm != nil {
		fmi := fm.mdb.NewIterator(slice)
		fmi.SetReleaser(&memdbReleaser{m: fm})
		i = append(i, fmi)
	}
	i = append(i, ti...)
	strict := opt.GetStrict(db.s.o.Options, ro, opt.StrictReader)
	mi := iterator.NewMergedIterator(i, db.s.icmp, strict)
	mi.SetReleaser(&versionReleaser{v: v})
	return mi
}

// NewIterator creates an iterator from the table.
//
// Slice allows slicing the iterator to only contains keys in the given
// range. A nil Range.Start is treated as a key before all keys in the
// table. And a nil Range.Limit is treated as a key after all keys in
// the table.
//
// The returned iterator is not goroutine-safe and should be released
// when not used.
//
// Also read Iterator documentation of the leveldb/iterator package.
func (r *Reader) NewIterator(slice *util.Range, ro *opt.ReadOptions) iterator.Iterator {
	r.mu.RLock()
	defer r.mu.RUnlock()

	if r.err != nil {
		return iterator.NewEmptyIterator(r.err)
	}

	fillCache := !ro.GetDontFillCache()
	indexBlock, rel, err := r.getIndexBlock(fillCache)
	if err != nil {
		return iterator.NewEmptyIterator(err)
	}
	index := &indexIter{
		blockIter: r.newBlockIter(indexBlock, rel, slice, true),
		tr:        r,
		slice:     slice,
		fillCache: !ro.GetDontFillCache(),
	}
	return iterator.NewIndexedIterator(index, opt.GetStrict(r.o, ro, opt.StrictReader))
}