[SPARK-19518][SQL] IGNORE NULLS in first / last in SQL

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to add IGNORE NULLS keyword in first/last in Spark's parser likewise http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions057.htm. This simply maps the keywords to existing ignoreNullsExpr.

Before

scala> sql("select first('a' IGNORE NULLS)").show()

org.apache.spark.sql.catalyst.parser.ParseException:
extraneous input 'NULLS' expecting {')', ','}(line 1, pos 24)

== SQL ==
select first('a' IGNORE NULLS)
------------------------^^^

  at org.apache.spark.sql.catalyst.parser.ParseException.withCommand(ParseDriver.scala:210)
  at org.apache.spark.sql.catalyst.parser.AbstractSqlParser.parse(ParseDriver.scala:112)
  at org.apache.spark.sql.execution.SparkSqlParser.parse(SparkSqlParser.scala:46)
  at org.apache.spark.sql.catalyst.parser.AbstractSqlParser.parsePlan(ParseDriver.scala:66)
  at org.apache.spark.sql.SparkSession.sql(SparkSession.scala:622)
  ... 48 elided

After

scala> sql("select first('a' IGNORE NULLS)").show()

+--------------+
|first(a, true)|
+--------------+
|             a|
+--------------+

How was this patch tested?

Unit tests in ExpressionParserSuite.

[HyukjinKwon]

@hvanhovell, could you check if it makes sense please?

[hvanhovell]

NIT: you don't have to check both (the grammar enforces that they both need to be present), either one suffices.

[HyukjinKwon]

Ah, sure. Thank you.

[hvanhovell]

LGTM - one minor comment

[SparkQA]

Test build #75606 has finished for PR 17566 at commit eeaa636.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #75607 has finished for PR 17566 at commit 6d21ad8.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[hvanhovell]

LGTM - merging to master. Thanks!

[HyukjinKwon]

Thank you @hvanhovell.