core/src/main/java/com/orientechnologies/orient/core/index/OIndexes.java
private static Set<String> getIndexTypes() {
    final Set<String> types = new HashSet<>();
    final Iterator<OIndexFactory> ite = getAllFactories();
    while (ite.hasNext()) {
      types.addAll(ite.next().getTypes());
    }
    return types;
  }
private static Set<String> getIndexTypes() {
    final Set<String> types = new HashSet<>();
    final Iterator<OIndexFactory> ite = getAllFactories();
    while (ite.hasNext()) {
      types.addAll(ite.next().getTypes());
    }
    return types;
  }
Iterates on all factories and append all index types.

@return Set of all index types.