为什么MySQL没有使用我的FULLTEXT索引？

Question

I have the following query:

EXPLAIN SELECT *
FROM glean2_saves
WHERE username = '1d85d5aed8b02b3d6b0c155a563293ef'
AND ses_id = 'e4fa3ae216f5033fbd16d6c66370954c'
AND save_status =1
ORDER BY id DESC 

And the result is this:

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   SIMPLE  glean2_saves    ref     save_status,username,ses_id     save_status     2   const   286315  Using where; Using filesort

Here are my indexes:

Action  Keyname Type    Unique  Packed  Column  Cardinality Collation   Null    Comment
Edit Edit   Drop Drop   PRIMARY BTREE   Yes No  id  331837  A       
Edit Edit   Drop Drop   save_status BTREE   No  No  save_status 3   A   YES 
Edit Edit   Drop Drop   nickname    FULLTEXT    No  No  nickname    7374        YES 
Edit Edit   Drop Drop   username    FULLTEXT    No  No  username    7717        YES 
Edit Edit   Drop Drop   ses_id  FULLTEXT    No  No  ses_id  11442       YES 

I do have indexes on the right columns, but why are they not used ? How come they are in the "possible keys" (save_status, username, ses_id) but only one "key" is actually used: save_status

Without the indexes, the query takes way too long (sometimes over 15 seconds), it should take less than one second. The database is over 330k entries.

Answer 1

For this query, you want one index that covers the where and order by clauses. That index would be:

glean2_saves(username, ses_id, save_status, id)

Note that because you are doing equality comparisons, the first three columns can be in any order. But the last column needs to be id for the order by .

As for why MySQL is not using the individual indexes. Let's just say that merging indexes for where clauses is a lot of work, and might result in more work than scanning one index and doing the tests for the remaining columns.