Github user gatorsmile commented on the issue:
https://github.com/apache/spark/pull/14580
```Scala
val df12 = df1.join(df2, $"df1.a" === $"df2.a", "fullouter")
.select(coalesce($"df1.b", $"df2.c").as("a"), $"df1.b", $"df2.c")
df12.join(df3, "a").explain(true)
```
This is an example to show that we should not eliminate the outer join,
even if `isnotnull(coalesce(b#227, c#238))` contains the attributes that are
not in join conditions.
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