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From "Jim Apple (Code Review)" <ger...@cloudera.org>
Subject [Impala-ASF-CR] IMPALA-4787: Optimize APPX MEDIAN() memory usage
Date Sat, 18 Feb 2017 05:54:41 GMT
Jim Apple has posted comments on this change.

Change subject: IMPALA-4787: Optimize APPX_MEDIAN() memory usage
......................................................................


Patch Set 5:

(1 comment)

http://gerrit.cloudera.org:8080/#/c/6025/5/be/src/exprs/aggregate-functions-ir.cc
File be/src/exprs/aggregate-functions-ir.cc:

PS5, Line 1122: ((double) state->source_size - r) / state->source_size
I'm not yet convinced this is correct. I'm not convinced it is correct in HEAD.

First, a nit: I think the denominator should be increased by 1. (I think the expected value
of the maximum of two independent random variables over the uniform distribution on (0,1)
is 2/3, not 3/4)

Second, while I understand that this produces values in the given range, I don't think I understand
how this implements the algorithm mentioned in the .h file, in which values of weight k are
mapped to r^(1/k), where r is uniform random from (0,1).


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Gerrit-MessageType: comment
Gerrit-Change-Id: I99adaad574d4fb0a3cf38c6cbad8b2a23df12968
Gerrit-PatchSet: 5
Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-Owner: Taras Bobrovytsky <tbobrovytsky@cloudera.com>
Gerrit-Reviewer: Alex Behm <alex.behm@cloudera.com>
Gerrit-Reviewer: Jim Apple <jbapple-impala@apache.org>
Gerrit-Reviewer: Marcel Kornacker <marcel@cloudera.com>
Gerrit-Reviewer: Matthew Jacobs <mj@cloudera.com>
Gerrit-Reviewer: Mostafa Mokhtar <mmokhtar@cloudera.com>
Gerrit-Reviewer: Taras Bobrovytsky <tbobrovytsky@cloudera.com>
Gerrit-HasComments: Yes

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