public WindowFunctionInfo getWindowFunctionInfo(String functionName) throws SemanticException {
    // First try without qualifiers - would resolve builtin/temp functions
    FunctionInfo info = getFunctionInfo(WINDOW_FUNC_PREFIX + functionName);
    // Try qualifying with current db name for permanent functions and try register function to session
    if (info == null && FunctionRegistry.getFunctionInfo(functionName) != null) {
      String qualifiedName = FunctionUtils.qualifyFunctionName(
              functionName, SessionState.get().getCurrentDatabase().toLowerCase());
      info = getFunctionInfo(WINDOW_FUNC_PREFIX + qualifiedName);
    }
    if (info instanceof WindowFunctionInfo) {
      return (WindowFunctionInfo) info;
    }
    return null;
  }
Looks up the function name in the registry. If enabled, will attempt to search the metastore
for the function.
@param functionName
@return
ql/src/java/org/apache/hadoop/hive/ql/exec/Registry.java