首先按照roachpb.Key的字节序顺序比较
其次，在有一个时间戳值为空时，按照hlc时间戳正序比较，否则，按照hlc时间戳逆序比较
如下排序结果：
key, hlc
123, 0
123, 0
123, 54
123, 24
123, 24
234, 0
234, 34
234, 24
234, 14源码分析：
pkg/storage/engine/mvcc.go
type MVCCKey struct {
Key roachpb.Key
Timestamp hlc.Timestamp
}c-deps/libroach/comparator.h
class DBComparator : public rocksdb::Comparator {
...
virtual int Compare(const rocksdb::Slice& a, const rocksdb::Slice& b) const override;
...
}c-deps/libroach/comparator.cc
int DBComparator::Compare(const rocksdb::Slice& a, const rocksdb::Slice& b) const {
rocksdb::Slice key_a, key_b;
rocksdb::Slice ts_a, ts_b;
if (!SplitKey(a, &key_a, &ts_a) || !SplitKey(b, &key_b, &ts_b)) {
// This should never happen unless there is some sort of corruption of
// the keys.
return a.compare(b);
}
const int c = key_a.compare(key_b);
if (c != 0) {
return c;
}
if (ts_a.empty()) {
if (ts_b.empty()) {
return 0;
}
return -1;
} else if (ts_b.empty()) {
return +1;
}
return ts_b.compare(ts_a);
}