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

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

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

## How was this patch tested?

Unit tests in `ExpressionParserSuite`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17566 from HyukjinKwon/SPARK-19518.

1 files changed, 8 insertions, 0 deletions

diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala
index d1c6b50536..e7f3b64a71 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala
@@ -21,6 +21,7 @@ import java.sql.{Date, Timestamp}
 import org.apache.spark.sql.catalyst.FunctionIdentifier
 import org.apache.spark.sql.catalyst.analysis.{UnresolvedAttribute, _}
 import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.expressions.aggregate.{First, Last}
 import org.apache.spark.sql.catalyst.plans.PlanTest
 import org.apache.spark.sql.types._
 import org.apache.spark.unsafe.types.CalendarInterval
@@ -549,4 +550,11 @@ class ExpressionParserSuite extends PlanTest {
     val complexName2 = FunctionIdentifier("ba``r", Some("fo``o"))
     assertEqual(complexName2.quotedString, UnresolvedAttribute("fo``o.ba``r"))
   }
+
+  test("SPARK-19526 Support ignore nulls keywords for first and last") {
+    assertEqual("first(a ignore nulls)", First('a, Literal(true)).toAggregateExpression())
+    assertEqual("first(a)", First('a, Literal(false)).toAggregateExpression())
+    assertEqual("last(a ignore nulls)", Last('a, Literal(true)).toAggregateExpression())
+    assertEqual("last(a)", Last('a, Literal(false)).toAggregateExpression())
+  }
 }