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From "Paul Rogers (JIRA)" <j...@apache.org>
Subject [jira] [Created] (DRILL-5019) ExternalSortBatch spills all batches to disk even if even one spills
Date Wed, 09 Nov 2016 00:28:58 GMT
Paul Rogers created DRILL-5019:

             Summary: ExternalSortBatch spills all batches to disk even if even one spills
                 Key: DRILL-5019
                 URL: https://issues.apache.org/jira/browse/DRILL-5019
             Project: Apache Drill
          Issue Type: Improvement
    Affects Versions: 1.8.0
            Reporter: Paul Rogers
            Priority: Minor

The ExternalSortBatch (ESB) operator sorts batches while spilling to disk to stay within a
defined memory budget.

Assume the memory budget is 10 GB. Assume that the actual volume of data to be sorted is 10.1
GB. The ESB spills the extra 0.1 GB to disk. (Actually spills more than that, say 5 GB.)

At the completion of the run, ESB has read all incoming batches. It must now merge those batches.
It does so by spilling **all** batches to disk, then doing a disk-based merge.

This means that exceeding the memory limit by even a small amount is the same as having a
very low memory limit: all batches must spill.

This solution is simple, it works, and has some amount of logic.

But, it would be better to have a slightly more advanced solution that spills only the smallest
possible set of batches to disk, then does a hybrid in-memory, on-disk merge, saving the unnecessary
write/read cycle.

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