func (txn *Txn) NewIterator(opt IteratorOptions) *Iterator {
	if txn.discarded {
		panic("Transaction has already been discarded")
	}
	// Do not change the order of the next if. We must track the number of running iterators.
	if atomic.AddInt32(&txn.numIterators, 1) > 1 && txn.update {
		atomic.AddInt32(&txn.numIterators, -1)
		panic("Only one iterator can be active at one time, for a RW txn.")
	}

	// TODO: If Prefix is set, only pick those memtables which have keys with
	// the prefix.
	tables, decr := txn.db.getMemTables()
	defer decr()
	txn.db.vlog.incrIteratorCount()
	var iters []y.Iterator
	if itr := txn.newPendingWritesIterator(opt.Reverse); itr != nil {
		iters = append(iters, itr)
	}
	for i := 0; i < len(tables); i++ {
		iters = append(iters, tables[i].NewUniIterator(opt.Reverse))
	}
	iters = txn.db.lc.appendIterators(iters, &opt) // This will increment references.
	res := &Iterator{
		txn:    txn,
		iitr:   y.NewMergeIterator(iters, opt.Reverse),
		opt:    opt,
		readTs: txn.readTs,
	}
	return res
}
func (txn *Txn) NewIterator(opt IteratorOptions) *Iterator {
	if txn.discarded {
		panic("Transaction has already been discarded")
	}
	// Do not change the order of the next if. We must track the number of running iterators.
	if atomic.AddInt32(&txn.numIterators, 1) > 1 && txn.update {
		atomic.AddInt32(&txn.numIterators, -1)
		panic("Only one iterator can be active at one time, for a RW txn.")
	}

	// TODO: If Prefix is set, only pick those memtables which have keys with
	// the prefix.
	tables, decr := txn.db.getMemTables()
	defer decr()
	txn.db.vlog.incrIteratorCount()
	var iters []y.Iterator
	if itr := txn.newPendingWritesIterator(opt.Reverse); itr != nil {
		iters = append(iters, itr)
	}
	for i := 0; i < len(tables); i++ {
		iters = append(iters, tables[i].NewUniIterator(opt.Reverse))
	}
	iters = txn.db.lc.appendIterators(iters, &opt) // This will increment references.
	res := &Iterator{
		txn:    txn,
		iitr:   y.NewMergeIterator(iters, opt.Reverse),
		opt:    opt,
		readTs: txn.readTs,
	}
	return res
}
// NewIterator returns a new iterator. Depending upon the options, either only keys, or both
// key-value pairs would be fetched. The keys are returned in lexicographically sorted order.
// Using prefetch is recommended if you're doing a long running iteration, for performance.
//
// Multiple Iterators:
// For a read-only txn, multiple iterators can be running simultaneously.  However, for a read-write
// txn, only one can be running at one time to avoid race conditions, because Txn is thread-unsafe.