code: public function createIndex($name, $table, $columns, $unique = false)
      {
          $sql = $this->db->getQueryBuilder()->createIndex($name, $table, $columns, $unique);
  
          return $this->setSql($sql)->requireTableSchemaRefresh($table);
      }
docs: Creates a SQL command for creating a new index.
  @param string $name the name of the index. The name will be properly quoted by the method.
  @param string $table the table that the new index will be created for. The table name will be properly quoted by the method.
  @param string|array $columns the column(s) that should be included in the index. If there are multiple columns, please separate them
  by commas. The column names will be properly quoted by the method.
  @param bool $unique whether to add UNIQUE constraint on the created index.
  @return $this the command object itself