[SPARK-27560] HashPartitioner uses Object.hashCode which is not seeded

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Not A Problem
• Affects Version/s: 2.4.0
• Component/s: Java API

Environment

Notebook is running spark v2.4.0 local[*]

Python 3.6.6 (default, Sep  6 2018, 13:10:03)
[GCC 4.2.1 Compatible Apple LLVM 9.1.0 (clang-902.0.39.2)] on darwin

I imagine this would reproduce on all operating systems and most versions of spark though.

• Assignee: Unassigned
• Reporter: Andrew McHarg
• Created: 24/Apr/19 21:21
• Updated: 10/Jul/19 14:56
• Resolved: 10/Jul/19 14:56

Description

Forgive the quality of the bug report here, I am a pyspark user and not super familiar with the internals of spark, yet it seems I have a strange corner case with the HashPartitioner.

This may already be known but repartition with HashPartitioner seems to assign everything the same partition if data that was partitioned by the same column is only partially read (say one partition). I suppose it is obvious concequence of Object.hashCode being deterministic but took some while to track down.

Steps to repro:

1. Get dataframe with a bunch of uuids say 10000
2. repartition(100, 'uuid_column')
3. save to parquet
4. read from parquet
5. collect()[:100] then filter using pyspark.sql.functions isin (yes I know this is bad and sampleBy should probably be used here)
6. repartition(10, 'uuid_column')
7. Resulting dataframe will have all of its data in one single partition

Jupyter notebook for the above: https://gist.github.com/robo-hamburger/4752a40cb643318464e58ab66cf7d23e

I think an easy fix would be to seed the HashPartitioner like many hashtable libraries do to avoid denial of service attacks. It also might be the case this is obvious behavior for more experienced spark users

Issue Links

• links to: GitHub Pull Request #25034