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From "Christopher Tubbs (JIRA)" <>
Subject [jira] [Commented] (ACCUMULO-4011) Allow combiners reduce to return null
Date Fri, 25 Sep 2015 17:36:04 GMT


Christopher Tubbs commented on ACCUMULO-4011:

Existing combiners are not allowed to return null here. So, relaxing this constraint and handling
this case could cause unexpected behavior to existing combiners, but only those which were
already incorrectly written.

> Allow combiners reduce to return null
> -------------------------------------
>                 Key: ACCUMULO-4011
>                 URL:
>             Project: Accumulo
>          Issue Type: Improvement
>            Reporter: Keith Turner
>             Fix For: 1.8.0
> Opening based on discussion from [PR 47|].
 If the reduce function in a combiner could return null to indicate that data should be dropped,
this could offer a solution to ACCUMULO-2232.
> For example assume someone was using a summing combiner and wanted to delete a column
with a current value of 18.  They could insert -18.  If the combiner returned null when everything
summed to 0, this would delete the row col.  This method of deleting data would safe during
partial compactions and would work correctly with new data being inserted after the -18 was
> If this done, needs to be done in a way that is compatible with existing combiners.

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