[jira] [Created] (DRILL-7457) Join assignment is random when table costa are identical

["Paul Rogers (Jira)" <ji...@apache.org>]

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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