Github user andrewor14 commented on a diff in the pull request:

    https://github.com/apache/spark/pull/12117#discussion_r58454910
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala
 ---
    @@ -428,53 +432,85 @@ class SessionCatalog(
        */
       def dropFunction(name: FunctionIdentifier): Unit = {
         val db = name.database.getOrElse(currentDb)
    +    val qualified = name.copy(database = Some(db)).unquotedString
    +    if (functionRegistry.functionExists(qualified)) {
    +      // If we have loaded this function into FunctionRegistry,
    +      // also drop it from there.
    +      functionRegistry.dropFunction(qualified)
    +    }
         externalCatalog.dropFunction(db, name.funcName)
       }
     
       /**
    -   * Alter a metastore function whose name that matches the one specified 
in `funcDefinition`.
    -   *
    -   * If no database is specified in `funcDefinition`, assume the function 
is in the
    -   * current database.
    -   *
    -   * Note: If the underlying implementation does not support altering a 
certain field,
    -   * this becomes a no-op.
    -   */
    -  def alterFunction(funcDefinition: CatalogFunction): Unit = {
    -    val db = funcDefinition.identifier.database.getOrElse(currentDb)
    -    val newFuncDefinition = funcDefinition.copy(
    -      identifier = FunctionIdentifier(funcDefinition.identifier.funcName, 
Some(db)))
    -    externalCatalog.alterFunction(db, newFuncDefinition)
    -  }
    -
    -  /**
        * Retrieve the metadata of a metastore function.
        *
        * If a database is specified in `name`, this will return the function 
in that database.
        * If no database is specified, this will return the function in the 
current database.
        */
    +  // TODO: have a better name. This method is actually for fetching the 
metadata of a function.
       def getFunction(name: FunctionIdentifier): CatalogFunction = {
         val db = name.database.getOrElse(currentDb)
         externalCatalog.getFunction(db, name.funcName)
       }
     
    +  /**
    +   * Check if a function is already existing.
    +   *
    +   */
    +  def functionExists(name: FunctionIdentifier): Boolean = {
    +    if (functionRegistry.functionExists(name.unquotedString)) {
    +      // This function exists in the FunctionRegistry.
    +      true
    +    } else {
    +      // Need to check if this function exists in the metastore.
    +      try {
    +        getFunction(name) != null
    +      } catch {
    +        case _: NoSuchFunctionException => false
    +        case _: AnalysisException => false // HiveExternalCatalog wraps 
all exceptions with it.
    --- End diff --
    
    This use of `try` is kind of hacky. The right thing to do here is to 
implement `functionExists` in the external catalog as well. That means we also 
need to implement it in `HiveClientImpl`. We can do that later if you prefer.


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