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Posted to reviews@spark.apache.org by GitBox <gi...@apache.org> on 2021/10/11 09:42:12 UTC
[GitHub] [spark] cloud-fan edited a comment on pull request #34034: [SPARK-36794][SQL] Ignore duplicated join keys when building relation for SEMI/ANTI hash join
cloud-fan edited a comment on pull request #34034:
URL: https://github.com/apache/spark/pull/34034#issuecomment-939862907
Unfortunately, this breaks broadcast reuse which causes perf regression. To reproduce
```
scala> val df1 = spark.range(1000)
df1: org.apache.spark.sql.Dataset[Long] = [id: bigint]
scala> val df2 = spark.range(100)
df2: org.apache.spark.sql.Dataset[Long] = [id: bigint]
scala> val j1 = df1.join(df2, Seq("id"), "inner")
j1: org.apache.spark.sql.DataFrame = [id: bigint]
scala> val j2 = df1.join(df2, Seq("id"), "left_semi")
j2: org.apache.spark.sql.DataFrame = [id: bigint]
scala> val res = j1.union(j2)
res: org.apache.spark.sql.Dataset[org.apache.spark.sql.Row] = [id: bigint]
scala> res.collect()
res0: Array[org.apache.spark.sql.Row] = Array([0], ...
scala> res.explain
```
Before this PR, the query plan was
```
AdaptiveSparkPlan isFinalPlan=true
+- == Final Plan ==
Union
:- *(3) Project [id#0L]
: +- *(3) BroadcastHashJoin [id#0L], [id#2L], Inner, BuildRight, false
: :- *(3) Range (0, 1000, step=1, splits=1)
: +- BroadcastQueryStage 0
: +- BroadcastExchange HashedRelationBroadcastMode(List(input[0, bigint, false]),false,false), [id=#79]
: +- *(1) Range (0, 100, step=1, splits=1)
+- *(4) BroadcastHashJoin [id#12L], [id#13L], LeftSemi, BuildRight, false
:- *(4) Range (0, 1000, step=1, splits=1)
+- BroadcastQueryStage 2
+- ReusedExchange [id#13L], BroadcastExchange HashedRelationBroadcastMode(List(input[0, bigint, false]),false,false), [id=#79]
```
Now it's
```
AdaptiveSparkPlan isFinalPlan=true
+- == Final Plan ==
Union
:- *(3) Project [id#0L]
: +- *(3) BroadcastHashJoin [id#0L], [id#2L], Inner, BuildRight, false
: :- *(3) Range (0, 1000, step=1, splits=1)
: +- BroadcastQueryStage 0
: +- BroadcastExchange HashedRelationBroadcastMode(List(input[0, bigint, false]),false,false), [id=#41]
: +- *(1) Range (0, 100, step=1, splits=1)
+- *(4) BroadcastHashJoin [id#6L], [id#7L], LeftSemi, BuildRight, false
:- *(4) Range (0, 1000, step=1, splits=1)
+- BroadcastQueryStage 1
+- BroadcastExchange HashedRelationBroadcastMode(List(input[0, bigint, false]),false,true), [id=#50]
+- *(2) Range (0, 100, step=1, splits=1)
```
Ignore duplicated key is a small improvement and broadcast reuse is definitely more important to the query performance. I'm reverting this first. Please re-propose this optimization without breaking broadcast reuse.
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