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Hive, mail # dev - Review Request: float and double calculation is inaccurate in Hive


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Johnny Zhang 2012-12-18, 00:10
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Re: Review Request: float and double calculation is inaccurate in Hive
Johnny Zhang 2012-12-18, 00:37

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This is an automatically generated e-mail. To reply, visit:
https://reviews.apache.org/r/8653/
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(Updated Dec. 18, 2012, 12:37 a.m.)
Review request for hive.
Description (updated)
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I found this during debug the e2e test failures. I found Hive miss calculate the float and double value. Take float calculation as an example:
hive> select f from all100k limit 1;
48308.98
hive> select f/10 from all100k limit 1;
4830.898046875 <--added 04875 in the end
hive> select f*1.01 from all100k limit 1;
48792.0702734375 <--should be 48792.0698
It might be essentially the same problem as http://effbot.org/pyfaq/why-are-floating-point-calculations-so-inaccurate.htm But since e2e test compare the results with mysql and seems mysql does it right, so it is worthy fixing it in Hive.
This addresses bug HIVE-3715.
    https://issues.apache.org/jira/browse/HIVE-3715
Diffs
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  http://svn.apache.org/repos/asf/hive/trunk/ql/src/java/org/apache/hadoop/hive/ql/udf/UDFOPDivide.java 1423224
  http://svn.apache.org/repos/asf/hive/trunk/ql/src/java/org/apache/hadoop/hive/ql/udf/UDFOPMultiply.java 1423224

Diff: https://reviews.apache.org/r/8653/diff/
Testing
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I did test to compare the result with mysql default float precision setting, the result is identical.

query:          select f, f*1.01, f/10 from all100k limit 1;
mysql result:   48309       48792.0702734375    4830.898046875
hive result:    48308.98    48792.0702734375 4830.898046875
I apply this patch and run the hive e2e test, and the tests all pass (without this patch, 5 related failures)
Thanks,

Johnny Zhang

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Mark Grover 2012-12-18, 00:38
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Johnny Zhang 2012-12-18, 01:13
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Bharath Mundlapudi 2012-12-18, 08:51
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Mark Grover 2012-12-21, 21:43
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Johnny Zhang 2012-12-18, 01:00
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Mark Grover 2012-12-21, 21:54