code: public function addIndex($strTableName, $strIndexType, $strColName)
      {
          $this->checkColumnExists($strTableName, $strColName);
          $this->connection->query(
              sprintf(
                  self::STATEMENT_ADD_INDEX_COLUMN,
                  $strTableName,
                  $strIndexType,
                  $strColName
              )
          );
      }
docs: Add a index to given tablename for specified columnname
  
  @param string $strTableName The table name.
  
  @param string $strIndexType The index type.
  
  @param string $strColName   The column name to add a index.
  
  @return void
  
  @throws \Exception If an invalid table name has been passed or the table does not exist, the column name is
  invalid or the column does not exist.