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https://issues.apache.org/jira/browse/SPARK-7712?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14554753#comment-14554753
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Herman van Hovell tot Westerflier commented on SPARK-7712:
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I have updated the description to explain the design.
> Native Spark Window Functions & Performance 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 tot Westerflier
> Fix For: 1.5.0
>
> Original Estimate: 336h
> Remaining Estimate: 336h
>
> 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.
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