[development] Drupal 7 Taxonomy CCK question

Hello, I have created some terms (vocab=test) in Taxonomy and linked them with a content type CCK using "Term reference". I found they are linked up in the DB by the table "taxonomy_index" and "field_data_field_test", why duplicated linkage (both table contains some data)? Seem taxonomy_index is not needed and data inside not incorrect, right? Is it a bug? Thanks.