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https://issues.apache.org/jira/browse/SPARK-7712?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Herman van Hovell updated SPARK-7712:
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Description: This is an umbrella ticket for Window Function Improvements
targetted at SPARK-1.5.0/1.6.0 (was: Hi All,
After playing with the current spark window implementation, I tried to take
this to next level. My main goal is/was to address the following issues: Native
Spark SQL & Performance.
*Native Spark SQL*
The current implementation uses Hive UDAFs as its aggregation mechanism. We try
to address the following issues by moving to a more 'native' Spark SQL
approach:
- Window functions require Hive. Some people (mostly by accident) use Spark SQL
without Hive. Usage of UDAFs is still supported though.
- Adding your own Aggregates requires you to write them in Hive instead of
native Spark SQL.
- Hive UDAFs are very well written and quite quick, but they are opaque in
processing and memory management; this makes them hard to optimize. By using
'Native' Spark SQL constructs we can actually do alot more optimization, for
example AggregateEvaluation style Window processing (this would require us to
move some of the code out of the AggregateEvaluation class into some Common
base class), or Tungten style memory management.
*Performance*
- Much better performance (10x) in running cases (e.g. BETWEEN UNBOUNDED
PRECEDING AND CURRENT ROW) and UNBOUDED FOLLOWING cases. The current
implementation in spark uses a sliding window approach in these cases. This
means that an aggregate is maintained for every row, so space usage is N (N
being the number of rows). This also means that all these aggregates all need
to be updated separately, this takes N*(N-1)/2 updates. The running case
differs from the Sliding case because we are only adding data to an aggregate
function (no reset is required), we only need to maintain one aggregate (like
in the UNBOUNDED PRECEDING AND UNBOUNDED case), update the aggregate for each
row, and get the aggregate value after each update. This is what the new
implementation does. This approach only uses 1 buffer, and only requires N
updates; I am currently working on data with window sizes of 500-1000 doing
running sums and this saves a lot of time. The CURRENT ROW AND UNBOUNDED
FOLLOWING case also uses this approach and the fact that aggregate operations
are communitative, there is one twist though it will process the input buffer
in reverse.
- Fewer comparisons in the sliding case. The current implementation determines
frame boundaries for every input row. The new implementation makes more use of
the fact that the window is sorted, maintains the boundaries, and only moves
them when the current row order changes. This is a minor improvement.
- A single Window node is able to process all types of Frames for the same
Partitioning/Ordering. This saves a little time/memory spent buffering and
managing partitions.
- A lot of the staging code is moved from the execution phase to the
initialization phase. Minor performance improvement, and improves readability
of the execution code.
The original work including some benchmarking code for the running case can be
here: https://github.com/hvanhovell/spark-window
A PR has been created, this is still work in progress, and can be found here:
https://github.com/apache/spark/pull/6278
Comments, feedback and other discussion is much appreciated.)
> Window Function Improvements
> ----------------------------
>
> Key: SPARK-7712
> URL: https://issues.apache.org/jira/browse/SPARK-7712
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 1.4.0
> Reporter: Herman van Hovell
> Priority: Critical
> Original Estimate: 336h
> Remaining Estimate: 336h
>
> This is an umbrella ticket for Window Function Improvements targetted at
> SPARK-1.5.0/1.6.0
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