Explanation provided by Boaz:
(As explained in the design document) The new "automatic spill" feature of the Hash-Join operator may cause (if spilling occurs) the rows from the left/probe side to be returned in a different order than their incoming order (due to splitting the rows into partitions).
Currently the Drill planner assumes that left-order is preserved by the Hash-Join operator; therefore if not changes, a query relying on that order may return wrong results (when the Hash-Join spills).
A fix is needed. Here are few options (ordered from the simpler down to the most complex):
- Change the order rule in the planner. Thus whenever an order is needed above (downstream) the Hash-Join, the planner would add a sort operator. That would be a big execution time waste.
- When the planner needs the left-order above the Hash-Join, it may assess the size of the right/build side (need statistics). If the right side is small enough, the planner would set an option for the runtime to avoid spilling, hence preserving the left-side order. In case spilling becomes necessary, the code would return an error (possibly with a message suggesting setting some special option and retrying; the special option would add a sort operator and allow the hash-join to spill).
- When generating the code for the fragment above the Hash-Join (where left-order should be maintained) - at code-gen time check if the hash-join below spilled, and if so, add a sort operator. (Nothing like that exists in Drill now, so it may be complicated).