Github user gatorsmile commented on the pull request:
https://github.com/apache/spark/pull/11632#issuecomment-194931904
I like the idea. If we do not change the current behavior, name ambiguity
is just confusing. Maybe we can accept it. However, more importantly, it could
generate a wrong result. See the example
```scala
val df1 = Seq((1, 3), (2, 1)).toDF("keyCol1", "keyCol2")
val df2 = Seq((1, 4), (2, 1)).toDF("keyCol1", "keyCol3")
val df3 = df1.join(df2, df1("keyCol1") ===
df2("keyCol1")).select(df1("keyCol1"), $"keyCol3")
df3.join(df1, df3("keyCol3") === df1("keyCol1")).show()
```
The above query returns an empty result set.
However, the correct result should be (2,1,1,3).
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