Timothy Farkas created DRILL-6089:
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Summary: Planner changes needed as Hash-Join spill breaks
left-side order
Key: DRILL-6089
URL: https://issues.apache.org/jira/browse/DRILL-6089
Project: Apache Drill
Issue Type: Improvement
Environment: 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).
Reporter: Timothy Farkas
Assignee: Timothy Farkas
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