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https://issues.apache.org/jira/browse/HADOOP-12217?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Steve Scaffidi updated HADOOP-12217:
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Description:
Because DoubleWritable.hashCode() is incorrect, using DoubleWritables as the
keys in a HashMap results in abysmal performance, due to hash code collisions.
I discovered this when testing the latest version of Hive and certain mapjoin
queries were exceedingly slow.
Evidently, Hive has its own wrapper/subclass around Hadoop's DoubleWritable
that overrode used to override hashCode() with a correct implementation, but
for some reason they recently removed that code, so it now uses the incorrect
hashCode() method inherited from Hadoop's DoubleWritable.
It appears that this bug has been there since DoubleWritable was created(wow!)
so I can understand if fixing it is impractical due to the possibility of
breaking things down-stream, but I can't think of anything that *should* break,
off the top of my head.
Searching JIRA, I found several related tickets, which may be useful for some
historical perspective: HADOOP-3061, HADOOP-3243, HIVE-511, HIVE-1629, HIVE-7041
was:
Because DoubleWritable.hashCode() is incorrect, using DoubleWritables as the
keys in a HashMap results in abysmal performance, due to hash code collisions.
I discovered this when testing the latest version of Hive and certain mapjoin
queries were exceedingly slow.
Evidently, Hive has its own wrapper/subclass around Hadoop's DoubleWritable
that overrode used to override hashCode() with a correct implementation, but
for some reason they recently removed that code, so it now uses the incorrect
hashCode() method inherited from Hadoop's DoubleWritable.
It appears that this bug has been there since DoubleWritable was created(!) so
I can understand if fixing it is impractical due to the possibility of breaking
things down-stream, but I can't think of anything that *should* break, off the
top of my head.
Searching JIRA, I found several related tickets, which may be useful for some
historical perspective: HADOOP-3061, HADOOP-3243, HIVE-511, HIVE-1629, HIVE-7041
> hashCode in DoubleWritable returns same value for many numbers
> --------------------------------------------------------------
>
> Key: HADOOP-12217
> URL: https://issues.apache.org/jira/browse/HADOOP-12217
> Project: Hadoop Common
> Issue Type: Bug
> Components: io
> Affects Versions: 0.18.0, 0.18.1, 0.18.2, 0.18.3, 0.19.0, 0.19.1, 0.20.0,
> 0.20.1, 0.20.2, 0.20.203.0, 0.20.204.0, 0.20.205.0, 1.0.0, 1.0.1, 1.0.2,
> 1.0.3, 1.0.4, 1.1.0, 1.1.1, 1.2.0, 0.21.0, 0.22.0, 0.23.0, 0.23.1, 0.23.3,
> 2.0.0-alpha, 2.0.1-alpha, 2.0.2-alpha, 0.23.4, 2.0.3-alpha, 0.23.5, 0.23.6,
> 1.1.2, 0.23.7, 2.1.0-beta, 2.0.4-alpha, 0.23.8, 1.2.1, 2.0.5-alpha, 0.23.9,
> 0.23.10, 0.23.11, 2.1.1-beta, 2.0.6-alpha, 2.2.0, 2.3.0, 2.4.0, 2.5.0, 2.4.1,
> 2.5.1, 2.5.2, 2.6.0, 2.7.0, 2.7.1
> Reporter: Steve Scaffidi
> Labels: easyfix
>
> Because DoubleWritable.hashCode() is incorrect, using DoubleWritables as the
> keys in a HashMap results in abysmal performance, due to hash code collisions.
> I discovered this when testing the latest version of Hive and certain mapjoin
> queries were exceedingly slow.
> Evidently, Hive has its own wrapper/subclass around Hadoop's DoubleWritable
> that overrode used to override hashCode() with a correct implementation, but
> for some reason they recently removed that code, so it now uses the incorrect
> hashCode() method inherited from Hadoop's DoubleWritable.
> It appears that this bug has been there since DoubleWritable was
> created(wow!) so I can understand if fixing it is impractical due to the
> possibility of breaking things down-stream, but I can't think of anything
> that *should* break, off the top of my head.
> Searching JIRA, I found several related tickets, which may be useful for some
> historical perspective: HADOOP-3061, HADOOP-3243, HIVE-511, HIVE-1629,
> HIVE-7041
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