Nathan Holland created SPARK-58677:
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Summary: Expand hash functionalities of XXH3
Key: SPARK-58677
URL: https://issues.apache.org/jira/browse/SPARK-58677
Project: Spark
Issue Type: Improvement
Components: PySpark, SQL
Affects Versions: 4.4.0
Reporter: Nathan Holland
This ticket expands on completing the areas where I did not go into enough
detail about the return types of SPARK-45900 as well as the functionality that
a modern hashing algorithm on Spark could implement.
The original intent was to capture a new hashing model through XXH3 that
allowed for high performance hashing using XXH3 with the API of the earlier
XXHash64 model, while developing SPARK-45900 I found some limitations to the
XXHash64 model which I have fixed up, but have not landed in SPARK-45900.
One difference in this model I am proposing is that SPARK-45900 was resolved by
returning a string, this is more consistent with the older hashing models but
has heavier overhead and prevents future optimisations that could occur by
introducing a FixedBinaryType (not in scope) that allows HotSpot (C
interpreters like Photon) to take advantage of SIMD instructions.
As XXH3 was built specifically around these speed ups over XXHash64 and Spark
4.4 is not out yet, I believe now would be the time to discuss what this API
should return - and either choose with a seperate API that returns the BINARY
128 bytes or make the change to the solution of SPARK-45900 to return a BINARY
type by default and let users use the "hex" function if they want the result as
a Binary String.
In addition one of the benefits of xxhash64 I have heard from customers over
the other API's is it's simple API to support multiple columns to conceptually
allow the hashing of columns without having to implement brittle solutions
(often something like concat_ws) to do so. I believe an expansion to the
functionality of the proposed Spark 4.4 solution of XXH3 should be seen as a
successor to the XXHash64 function and this user friendly API.
My current codebase can be found here for a proposed implementation of the
above two key changes (better support for hashing data types and multiple
columns, and the return of a binary type which better captures the hash results
rather than a string)
*
[https://github.com/NathanNZ/spark/commit/d4317b9853a9b1f775c1ab11534dc8ea5a048040]
* I've also got a work in progress repository to demonstrate the improvements
of such a solution over the older hashing methods discussed in this repository,
with pre-baked containers to test the functionality.
** https://github.com/NathanNZ/spark-nathan-containers
Links:
- https://issues.apache.org/jira/browse/SPARK-45900 (Original SPARK-45900
ticket)
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