public function index()
    {
        $index = $this->getIndexName();

        $this->assertTrue(
            $this->table->hasIndex($index), "The {$this->name} column is not indexed."
        );

        $this->assertTrue(
            $this->table->getIndex($index)->isSimpleIndex(), "The {$this->name} column is not a simple index."
        );
    }