Affects Version/s: None
Fix Version/s: 4.8.0
There's a corner case I found where a DISTINCT and GROUP BY query along a prefix of a compound row key might return incorrect results.
The filter relies on seeing the _0 column absolutely last, and not seeing all Cells that should be filtered. That break in two scenarios:
- we have a table with key (key1, key2, key3) and columns (c1 and c2). Now construct a WHERE <a clause that always matches c1>, <a clause that filters by c2) GROUP BY key1, key2. Now the filter would mis-skip when it sees the Cell for c1.
- we force lower key column names. In that case those would sort after the _0 column. The DistinctPrefixFilter would see the _0 column first and skip.
In both case we are effectively changing the order in which the filters are applied. The DistinctPrefixFilter is no longer for the row.
I can fix #1 (by ignoring all Cells other than then _0 one). I do not know how to fix case #2.
I think this is a blocker and we may have to undo the entire DISTINCT and GROUP BY prefix optimization.