Assume a cloumn's max versions is 3, then we write 4 versions of this column. The oldest version doesn't remove immediately. But from the user view, the oldest version has gone. When user use a filter to query, if the filter skip a new version, then the oldest version will be seen again. But after compact the region, then the oldest version will never been seen. So it is weird for user. The query will get inconsistent result before and after region compaction.
The reason is matchColumn method of UserScanQueryMatcher. It first check the cell by filter, then check the number of versions needed. So if the filter skip the new version, then the oldest version will be seen again when it is not removed.
Have a discussion offline with Duo Zhang and Honghua Feng, now we have two solution for this problem. The first idea is check the number of versions first, then check the cell by filter. As the comment of setFilter, the filter is called after all tests for ttl, column match, deletes and max versions have been run.
But this idea has another problem, if a column's max version is 5 and the user query only need 3 versions. It first check the version's number, then check the cell by filter. So the cells number of the result may less than 3. But there are 2 versions which don't read anymore.
So the second idea has three steps.
1. check by the max versions of this column
2. check the kv by filter
3. check the versions which user need.
But this will lead the ScanQueryMatcher more complicated. And this will break the javadoc of Query.setFilter.
Now we don't have a final solution for this problem. Suggestions are welcomed.