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Posted to issues@spark.apache.org by "Hyukjin Kwon (JIRA)" <ji...@apache.org> on 2019/05/21 04:16:22 UTC
[jira] [Resolved] (SPARK-18591) Replace hash-based aggregates with
sort-based ones if inputs already sorted
[ https://issues.apache.org/jira/browse/SPARK-18591?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Hyukjin Kwon resolved SPARK-18591.
----------------------------------
Resolution: Incomplete
> Replace hash-based aggregates with sort-based ones if inputs already sorted
> ---------------------------------------------------------------------------
>
> Key: SPARK-18591
> URL: https://issues.apache.org/jira/browse/SPARK-18591
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 2.0.2
> Reporter: Takeshi Yamamuro
> Priority: Major
> Labels: bulk-closed
>
> Spark currently uses sort-based aggregates only in limited condition; the cases where spark cannot use partial aggregates and hash-based ones.
> However, if input ordering has already satisfied the requirements of sort-based aggregates, it seems sort-based ones are faster than the other.
> {code}
> ./bin/spark-shell --conf spark.sql.shuffle.partitions=1
> val df = spark.range(10000000).selectExpr("id AS key", "id % 10 AS value").sort($"key").cache
> def timer[R](block: => R): R = {
> val t0 = System.nanoTime()
> val result = block
> val t1 = System.nanoTime()
> println("Elapsed time: " + ((t1 - t0 + 0.0) / 1000000000.0)+ "s")
> result
> }
> timer {
> df.groupBy("key").count().count
> }
> // codegen'd hash aggregate
> Elapsed time: 7.116962977s
> // non-codegen'd sort aggregarte
> Elapsed time: 3.088816662s
> {code}
> If codegen'd sort-based aggregates are supported in SPARK-16844, this seems to make the performance gap bigger;
> {code}
> - codegen'd sort aggregate
> Elapsed time: 1.645234684s
> {code}
> Therefore, it'd be better to use sort-based ones in this case.
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