core/src/main/java/com/orientechnologies/orient/core/sql/OSQLEngine.java
public static Set<String> getFunctionNames() {
    final Set<String> types = new HashSet<String>();
    final Iterator<OSQLFunctionFactory> ite = getFunctionFactories();
    while (ite.hasNext()) {
      types.addAll(ite.next().getFunctionNames());
    }
    return types;
  }
public static Set<String> getFunctionNames() {
    final Set<String> types = new HashSet<String>();
    final Iterator<OSQLFunctionFactory> ite = getFunctionFactories();
    while (ite.hasNext()) {
      types.addAll(ite.next().getFunctionNames());
    }
    return types;
  }
Iterates on all factories and append all function names.

@return Set of all function names.