[jira] [Created] (DRILL-5912) Hash Join Enhancement: Avoid copying probe side values

["Boaz Ben-Zvi (JIRA)" <ji...@apache.org>]

Boaz Ben-Zvi created DRILL-5912:
-----------------------------------

             Summary: Hash Join Enhancement: Avoid copying probe side values
                 Key: DRILL-5912
                 URL: https://issues.apache.org/jira/browse/DRILL-5912
             Project: Apache Drill
          Issue Type: Improvement
          Components: Execution - Relational Operators
    Affects Versions: 1.11.0
            Reporter: Boaz Ben-Zvi
            Priority: Minor


When the Hash Join Operator (inner, or left outer) performs the "probe and project" task, it copies each probe side values to be projected. Example:
{code}
    public void projectProbeRecord(int probeIndex, int outIndex)
        throws SchemaChangeException
    {
        {
            vv15 .copyFromSafe((probeIndex), (outIndex), vv12);
        }
        {
            vv21 .copyFromSafe((probeIndex), (outIndex), vv18);
        }
    }
{code}

In the case where there are no duplicate-key entries in the build side, and no spilling took place, then each of the outer values is projected exactly once (for left outer), or at most once (for inner join). 

In such (common) cases, we could avoid the above copy, and just transfer the value vectors as is (or add a Selection Vector 2 for the inner join, to eliminate the unmatched entries).

This can be a significant performance enhancement, as copying each set of values is much more expensive than transposing vectors (e.g., perform the copy 64K times, plus allocation of the vectors, and possible resizing for variable sized types).




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