Github user lins05 commented on the issue:
https://github.com/apache/spark/pull/15487
@HyukjinKwon I've updated the usage string. Now it looks like this:
```
spark-sql> describe function first;
Function: first
Class: org.apache.spark.sql.catalyst.expressions.aggregate.First
Usage:
first(expr) - Returns the first value of `child` for a group of rows.
first(expr, isIgnoreNull=false) - Returns the first value of `child`
for a group of rows.
If isIgnoreNull is true, returns only non-null values.
Note that in most cases the result is nondeterministic.
spark-sql> describe function last;
Function: last
Class: org.apache.spark.sql.catalyst.expressions.aggregate.Last
Usage:
last(expr) - Returns the last value of `child` for a group of rows.
last(expr, isIgnoreNull) - Returns the last value of `child` for a
group of rows.
If isIgnoreNull is true, returns only non-null values.
Note that in most cases the result is nondeterministic.
```
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