[HADOOP-12217] hashCode in DoubleWritable returns same value for many numbers

• Type: Bug
• Status: Patch Available
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 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
• Component/s: io
• Labels: easyfix
• Assignee: Unassigned
• Reporter: Steve Scaffidi
• Created: 10/Jul/15 17:46
• Updated: 10/Aug/15 08:20

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

Issue Links

• relates to: HIVE-11502 Map side aggregation is extremely slow