Paul Rogers created DRILL-7457:
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Summary: Join assignment is random when table costa are identical
Key: DRILL-7457
URL: https://issues.apache.org/jira/browse/DRILL-7457
Project: Apache Drill
Issue Type: Bug
Reporter: Paul Rogers
Create a simple test: a join between two identical scans, call them t1 and t2.
Ensure that the scans report the same cost. Capture the logical plan. Repeat
the exercise several times. You will see that Drill randomly assigns t1 to the
left side or right side.
Operationally this might not make a difference. But, in tests, it means that
trying to compare an "actual" and "golden" plan is impossible as the plans are
unstable.
Also, if only the estimates are the same, but the table size differs, then
runtime performance will randomly be better on some query runs than others.
Better is to fall back to SQL statement table order if the two tables are
otherwise identical in cost.
This may be a Calcite issue rather than a Drill issue.
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