Simple query, still Using temporary; Using filesort ?!

[stacionari]

HI,

My query is like this:

SELECT tag.* FROM tag LEFT JOIN book_item ON tag.Tag_ID = book_item.BookItem_ID WHERE tag.Tag = "tags" GROUP BY tag.tag_ID

Indexes:

Table 'tag':
Keyname      Type      Cardinality      Field
PRIMARY     PRIMARY      203      Tag_ID
Tag                INDEX     203        Tag
Item_ID       INDEX     29      Item_ID
 
Table 'book_item':
Keyname      Type      Cardinality      Field
PRIMARY     PRIMARY     40      BookItem_ID

Explain:

id     select_type     table     type     possible_keys     key     key_len     ref     rows     Extra
1     SIMPLE     tag     ref     Tag     Tag     92     const     6     Using where; Using temporary; Using filesort
1     SIMPLE     book_item     eq_ref     PRIMARY     PRIMARY     4     makeliterature.tag.Tag_ID     1     Using index

My question is: how can I optimize this query to get rid of Using temporary; Using filesort ?

Thank you.