c21 opened a new pull request #34612:
URL: https://github.com/apache/spark/pull/34612
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### What changes were proposed in this pull request?
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FULL OUTER sort merge join (non-code-gen path) [copies join keys and buffers
input rows, even when rows from both sides do have matched
keys](https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/SortMergeJoinExec.scala#L1637-L1641).
This is unnecessary, as we can just output the row with smaller join keys, and
only buffer when both sides have matched keys. This would save us from
unnecessary copy and buffer, when both join sides have a lot of rows not
matched with each other.
### Why are the changes needed?
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Improve query performance for FULL OUTER sort merge join when code-gen is
disabled.
This would benefit query when both sides have a lot of rows not matched, and
join key is big in terms of size (e.g. string type).
Example micro benchmark:
```
def sortMergeJoin(): Unit = {
val N = 2 << 20
codegenBenchmark("sort merge join", N) {
val df1 = spark.range(N).selectExpr(s"cast(id * 15485863 as string) as
k1")
val df2 = spark.range(N).selectExpr(s"cast(id * 15485867 as string) as
k2")
val df = df1.join(df2, col("k1") === col("k2"), "full_outer")
assert(df.queryExecution.sparkPlan.find(_.isInstanceOf[SortMergeJoinExec]).isDefined)
df.noop()
}
}
```
Seeing run-time improvement over 60%:
```
Running benchmark: sort merge join
Running case: sort merge join without optimization
Stopped after 5 iterations, 10026 ms
Running case: sort merge join with optimization
Stopped after 5 iterations, 5954 ms
Java HotSpot(TM) 64-Bit Server VM 1.8.0_181-b13 on Mac OS X 10.16
Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
sort merge join: Best Time(ms) Avg Time(ms)
Stdev(ms) Rate(M/s) Per Row(ns) Relative
------------------------------------------------------------------------------------------------------------------------
sort merge join without optimization 1807 2005
157 1.2 861.4 1.0X
sort merge join with optimization 1135 1191
62 1.8 541.1 1.6X
```
### Does this PR introduce _any_ user-facing change?
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No.
### How was this patch tested?
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Existing unit tests e.g. `OuterJoinSuite.scala`.
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