Github user aray commented on the pull request:
https://github.com/apache/spark/pull/7841#issuecomment-150745807
@rxin, Not requiring the values would necessitate doing a separate query
for the distinct values of the column before the pivot query. It looks like at
least some DF operations (eg, drop) would need the result so even if we made
Pivot.output lazy we would be running an unexpected job.
If a user really didn't want to specify the values, they can explicitly do
the query:
```scala
df.groupBy("A", "B").pivot("C",
df.select("C").distinct.collect.map(_.getString(0)): _*).sum("D")
```
Needing to know the output columns of an operator for analysis/planning is
probably why the other SQL implementations require the values also (technically
Oracle supports omitting it but only in XML mode where you essentially just get
one column).
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