[SPARK-11768][SPARK-9196][SQL] Support now function in SQL (alias for current_timestamp).

This patch adds an alias for current_timestamp (now function).

Also fixes SPARK-9196 to re-enable the test case for current_timestamp.

Author: Reynold Xin <rxin@databricks.com>

Closes #9753 from rxin/SPARK-11768.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -244,6 +244,7 @@ object FunctionRegistry {
     expression[AddMonths]("add_months"),
     expression[CurrentDate]("current_date"),
     expression[CurrentTimestamp]("current_timestamp"),
+    expression[CurrentTimestamp]("now"),
     expression[DateDiff]("datediff"),
     expression[DateAdd]("date_add"),
     expression[DateFormatClass]("date_format"),

sql/core/src/test/scala/org/apache/spark/sql/DateFunctionsSuite.scala

@@ -38,15 +38,21 @@ class DateFunctionsSuite extends QueryTest with SharedSQLContext {
     assert(d0 <= d1 && d1 <= d2 && d2 <= d3 && d3 - d0 <= 1)
   }
 
-  // This is a bad test. SPARK-9196 will fix it and re-enable it.
-  ignore("function current_timestamp") {
+  test("function current_timestamp and now") {
     val df1 = Seq((1, 2), (3, 1)).toDF("a", "b")
     checkAnswer(df1.select(countDistinct(current_timestamp())), Row(1))
+
     // Execution in one query should return the same value
-    checkAnswer(sql("""SELECT CURRENT_TIMESTAMP() = CURRENT_TIMESTAMP()"""),
-      Row(true))
-    assert(math.abs(sql("""SELECT CURRENT_TIMESTAMP()""").collect().head.getTimestamp(
-      0).getTime - System.currentTimeMillis()) < 5000)
+    checkAnswer(sql("""SELECT CURRENT_TIMESTAMP() = CURRENT_TIMESTAMP()"""), Row(true))
+
+    // Current timestamp should return the current timestamp ...
+    val before = System.currentTimeMillis
+    val got = sql("SELECT CURRENT_TIMESTAMP()").collect().head.getTimestamp(0).getTime
+    val after = System.currentTimeMillis
+    assert(got >= before && got <= after)
+
+    // Now alias
+    checkAnswer(sql("""SELECT CURRENT_TIMESTAMP() = NOW()"""), Row(true))
   }
 
   val sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")