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Posted to issues@spark.apache.org by "Josh Rosen (JIRA)" <ji...@apache.org> on 2019/04/26 02:10:00 UTC
[jira] [Updated] (SPARK-27573) Collapse adjacent physical aggregate
operators when possible
[ https://issues.apache.org/jira/browse/SPARK-27573?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Josh Rosen updated SPARK-27573:
-------------------------------
Summary: Collapse adjacent physical aggregate operators when possible (was: Collapse adjacent aggregate physical operators when possible)
> Collapse adjacent physical aggregate operators when possible
> ------------------------------------------------------------
>
> Key: SPARK-27573
> URL: https://issues.apache.org/jira/browse/SPARK-27573
> Project: Spark
> Issue Type: Improvement
> Components: Optimizer, SQL
> Affects Versions: 2.4.0
> Reporter: Josh Rosen
> Priority: Major
>
> When an aggregation requires a shuffle, Spark SQL performs separate partial and final aggregations:
> {code:java}
> sql("select id % 100 as k, id as v from range(100000)")
> .groupBy("k")
> .sum("v")
> .explain
> == Physical Plan ==
> *(2) HashAggregate(keys=[k#64L], functions=[sum(v#65L)])
> +- Exchange(coordinator id: 2031684357) hashpartitioning(k#64L, 5340), coordinator[target post-shuffle partition size: 67108864]
> +- *(1) HashAggregate(keys=[k#64L], functions=[partial_sum(v#65L)])
> +- *(1) Project [(id#66L % 100) AS k#64L, id#66L AS v#65L]
> +- *(1) Range (0, 100000, step=1, splits=10)
> {code}
> However, consider what happens if the dataset being aggregated is already pre-partitioned by the aggregate's grouping columns:
> {code:java}
> sql("select id % 100 as k, id as v from range(100000)")
> .repartition(10, $"k")
> .groupBy("k")
> .sum("v")
> .explain
> == Physical Plan ==
> *(2) HashAggregate(keys=[k#50L], functions=[sum(v#51L)], output=[k#50L, sum(v)#58L])
> +- *(2) HashAggregate(keys=[k#50L], functions=[partial_sum(v#51L)], output=[k#50L, sum#63L])
> +- Exchange(coordinator id: 39015877) hashpartitioning(k#50L, 10), coordinator[target post-shuffle partition size: 67108864]
> +- *(1) Project [(id#52L % 100) AS k#50L, id#52L AS v#51L]
> +- *(1) Range (0, 100000, step=1, splits=10)
> {code}
> Here, we end up with back-to-back HashAggregate operators which are performed as part of the same stage.
> For certain aggregates (e.g. _sum_, _count_), this duplication is unnecessary: we could have just performed a total aggregation instead!
> The duplicate aggregate is problematic in cases where the aggregate inputs and outputs are the same order of magnitude (e.g.counting the number of duplicate records in a dataset where duplicates are extremely rare).
> My motivation for this optimization is similar to SPARK-1412: I know that partial aggregation doesn't help for my workload, so I want to somehow coerce Spark into skipping the ineffective partial aggregation and jumping directly to total aggregation.
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