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From "Keith Turner (JIRA)" <>
Subject [jira] [Commented] (ACCUMULO-4011) Allow combiners reduce to return null
Date Fri, 25 Sep 2015 18:00:05 GMT


Keith Turner commented on ACCUMULO-4011:

Yeah if something would have previously failed and now deletes stuff (when thats not the users
intention) that is not a good thing.  We could add a config option to the combiner that determines
how it deals with null return types.   By default it would fail when null is retuned, but
it could optionally be configured to delete data when null is returned.

> 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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