seekForPrev not suppoert ReadOptions'setIterStartSeqnum()

[Cicizz]

when i use seekForPrev,i want to skip some key,but setIterStartSeqnum() method is not valid.

[jay-zhuang]

What do you mean SetIterStartSeqnum() is not valid?
I tested cpp code, seems SeekForPrev() doesn't work well with ReadOptions.iter_start_seqnum, here is a test which throws an assert exception:

TEST_P(DBIteratorTest, IterSeekForPrevBeforeNext) {
  ASSERT_OK(Put("a", "b"));
  auto seq = db_->GetLatestSequenceNumber();
  ASSERT_OK(Put("c", "d"));
  EXPECT_OK(dbfull()->Flush(FlushOptions()));
  ASSERT_OK(Put("0", "f"));
  ASSERT_OK(Put("1", "h"));
  EXPECT_OK(dbfull()->Flush(FlushOptions()));
  ASSERT_OK(Put("2", "j"));
  auto ro = ReadOptions();
  ro.iter_start_seqnum = seq;
  auto iter = NewIterator(ro);
  iter->SeekForPrev(Slice("0"));
  if (iter->Valid()) {
    std::cout << iter->key().ToString() << std::endl; // exception
  }
  delete iter;
}

https://github.com/jay-zhuang/rocksdb/blob/089e9af7a15404096bce60c6d210781d52f5d4d8/db/db_iterator_test.cc#L216

[jay-zhuang]

Currently iter_start_seqnum feature is not well maintained, as an alternative, you can use user-defined timestamp feature to filter out the data, which is much more flexible to use: https://github.com/facebook/rocksdb/wiki/User-Timestamp-%28Experimental%29