[SPARK-14910][SQL] Native DDL Command Support for Describe Function in Non-identifier Format

#### What changes were proposed in this pull request?
The existing `Describe Function` only support the function name in `identifier`. This is different from what Hive behaves. That is why many test cases `udf_abc` in `HiveCompatibilitySuite` are not using our native DDL support. For example,
- udf_not.q
- udf_bitwise_not.q

This PR is to resolve the issues. Now, we can support the command of `Describe Function` whose function names are in the following format:
- `qualifiedName` (e.g., `db.func1`)
- `STRING` (e.g., `'func1'`)
- `comparisonOperator` (e.g,. `<`)
- `arithmeticOperator` (e.g., `+`)
- `predicateOperator` (e.g., `or`)

Note, before this PR, we only have a native command support when the function name is in the format of `qualifiedName`.
#### How was this patch tested?
Added test cases in `DDLSuite.scala`. Also manually verified all the related test cases in `HiveCompatibilitySuite` passed.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #12679 from gatorsmile/descFunction.

3 files changed, 28 insertions, 3 deletions

diff --git a/sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4 b/sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4
index fa4b8c4868..64f68c9e9e 100644
--- a/sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4
+++ b/sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4
@@ -107,7 +107,7 @@ statement
     | SHOW TBLPROPERTIES table=tableIdentifier
         ('(' key=tablePropertyKey ')')?                                #showTblProperties
     | SHOW FUNCTIONS (LIKE? (qualifiedName | pattern=STRING))?         #showFunctions
-    | (DESC | DESCRIBE) FUNCTION EXTENDED? qualifiedName               #describeFunction
+    | (DESC | DESCRIBE) FUNCTION EXTENDED? describeFuncName            #describeFunction
     | (DESC | DESCRIBE) option=(EXTENDED | FORMATTED)?
         tableIdentifier partitionSpec? describeColName?                #describeTable
     | (DESC | DESCRIBE) DATABASE EXTENDED? identifier                  #describeDatabase
@@ -220,6 +220,14 @@ partitionVal
     : identifier (EQ constant)?
     ;
 
+describeFuncName
+    : qualifiedName
+    | STRING
+    | comparisonOperator
+    | arithmeticOperator
+    | predicateOperator
+    ;
+
 describeColName
     : identifier ('.' (identifier | STRING))*
     ;
@@ -519,6 +527,14 @@ comparisonOperator
     : EQ | NEQ | NEQJ | LT | LTE | GT | GTE | NSEQ
     ;
 
+arithmeticOperator
+    : PLUS | MINUS | ASTERISK | SLASH | PERCENT | DIV | TILDE | AMPERSAND | PIPE | HAT
+    ;
+
+predicateOperator
+    : OR | AND | IN | NOT
+    ;
+
 booleanValue
     : TRUE | FALSE
     ;
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala
index dd3577063f..1bada2ce67 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala
@@ -363,6 +363,7 @@ object FunctionRegistry {
     expression[Not]("not"),
     expression[Or]("or"),
 
+    // comparison operators
     expression[EqualNullSafe]("<=>"),
     expression[EqualTo]("="),
     expression[EqualTo]("=="),
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala
index 3d7b888d72..7f98c21af2 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala
@@ -112,8 +112,16 @@ class AstBuilder extends SqlBaseBaseVisitor[AnyRef] with Logging {
    * Create a plan for a DESCRIBE FUNCTION command.
    */
   override def visitDescribeFunction(ctx: DescribeFunctionContext): LogicalPlan = withOrigin(ctx) {
-    val functionName = ctx.qualifiedName().identifier().asScala.map(_.getText).mkString(".")
-    DescribeFunction(functionName, ctx.EXTENDED != null)
+    import ctx._
+    val functionName =
+      if (describeFuncName.STRING() != null) {
+        string(describeFuncName.STRING())
+      } else if (describeFuncName.qualifiedName() != null) {
+        describeFuncName.qualifiedName().identifier().asScala.map(_.getText).mkString(".")
+      } else {
+        describeFuncName.getText
+      }
+    DescribeFunction(functionName, EXTENDED != null)
   }
 
   /**