[SPARK-20349][SQL] ListFunctions returns duplicate functions after using persistent functions

### What changes were proposed in this pull request?
The session catalog caches some persistent functions in the `FunctionRegistry`, so there can be duplicates. Our Catalog API `listFunctions` does not handle it.

It would be better if `SessionCatalog` API can de-duplciate the records, instead of doing it by each API caller. In `FunctionRegistry`, our functions are identified by the unquoted string. Thus, this PR is try to parse it using our parser interface and then de-duplicate the names.

### How was this patch tested?
Added test cases.

Author: Xiao Li <gatorsmile@gmail.com>

Closes #17646 from gatorsmile/showFunctions.

1 files changed, 17 insertions, 0 deletions

diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveUDFSuite.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveUDFSuite.scala
index 4bbf925919..4446af2e75 100644
--- a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveUDFSuite.scala
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveUDFSuite.scala
@@ -573,6 +573,23 @@ class HiveUDFSuite extends QueryTest with TestHiveSingleton with SQLTestUtils {
       checkAnswer(testData.selectExpr("statelessUDF() as s").agg(max($"s")), Row(1))
     }
   }
+
+  test("Show persistent functions") {
+    val testData = spark.sparkContext.parallelize(StringCaseClass("") :: Nil).toDF()
+    withTempView("inputTable") {
+      testData.createOrReplaceTempView("inputTable")
+      withUserDefinedFunction("testUDFToListInt" -> false) {
+        val numFunc = spark.catalog.listFunctions().count()
+        sql(s"CREATE FUNCTION testUDFToListInt AS '${classOf[UDFToListInt].getName}'")
+        assert(spark.catalog.listFunctions().count() == numFunc + 1)
+        checkAnswer(
+          sql("SELECT testUDFToListInt(s) FROM inputTable"),
+          Seq(Row(Seq(1, 2, 3))))
+        assert(sql("show functions").count() == numFunc + 1)
+        assert(spark.catalog.listFunctions().count() == numFunc + 1)
+      }
+    }
+  }
 }
 
 class TestPair(x: Int, y: Int) extends Writable with Serializable {