[SPARK-8883][SQL]Remove the OverrideFunctionRegistry

Remove the `OverrideFunctionRegistry` from the Spark SQL, as the subclasses of `FunctionRegistry` have their own way to the delegate to the right underlying `FunctionRegistry`.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #7260 from chenghao-intel/override and squashes the following commits:

164d093 [Cheng Hao] enable the function registry
2ca8459 [Cheng Hao] remove the OverrideFunctionRegistry

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

@@ -35,19 +35,6 @@ trait FunctionRegistry {
   def lookupFunction(name: String, children: Seq[Expression]): Expression
 }
 
-class OverrideFunctionRegistry(underlying: FunctionRegistry) extends FunctionRegistry {
-
-  private val functionBuilders = StringKeyHashMap[FunctionBuilder](caseSensitive = false)
-
-  override def registerFunction(name: String, builder: FunctionBuilder): Unit = {
-    functionBuilders.put(name, builder)
-  }
-
-  override def lookupFunction(name: String, children: Seq[Expression]): Expression = {
-    functionBuilders.get(name).map(_(children)).getOrElse(underlying.lookupFunction(name, children))
-  }
-}
-
 class SimpleFunctionRegistry extends FunctionRegistry {
 
   private val functionBuilders = StringKeyHashMap[FunctionBuilder](caseSensitive = false)

sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala

@@ -139,8 +139,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
 
   // TODO how to handle the temp function per user session?
   @transient
-  protected[sql] lazy val functionRegistry: FunctionRegistry =
-    new OverrideFunctionRegistry(FunctionRegistry.builtin)
+  protected[sql] lazy val functionRegistry: FunctionRegistry = FunctionRegistry.builtin
 
   @transient
   protected[sql] lazy val analyzer: Analyzer =

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala

@@ -371,7 +371,7 @@ class HiveContext(sc: SparkContext) extends SQLContext(sc) {
   // Note that HiveUDFs will be overridden by functions registered in this context.
   @transient
   override protected[sql] lazy val functionRegistry: FunctionRegistry =
-    new OverrideFunctionRegistry(new HiveFunctionRegistry(FunctionRegistry.builtin))
+    new HiveFunctionRegistry(FunctionRegistry.builtin)
 
   /* An analyzer that uses the Hive metastore. */
   @transient

sql/hive/src/main/scala/org/apache/spark/sql/hive/hiveUDFs.scala

@@ -77,7 +77,7 @@ private[hive] class HiveFunctionRegistry(underlying: analysis.FunctionRegistry)
   }
 
   override def registerFunction(name: String, builder: FunctionBuilder): Unit =
-    throw new UnsupportedOperationException
+    underlying.registerFunction(name, builder)
 }
 
 private[hive] case class HiveSimpleUDF(funcWrapper: HiveFunctionWrapper, children: Seq[Expression])