Simple heavily-indexed table slow query in MySQL - mysql

Question

I am having troubles with a particular query being slow. Although everything is heavily indexed, some similar queries working fine and the indexes are used, the query still is slow as hell. I cannot understand why, so maybe anybody can help.
Just for the prerequisites: the write speed of the underlying table does not matter. The table contains ~3.5 million entries but I guess MySQL should handle that just fine.
The query that is being slow takes about 2s
SELECT DISTINCT t.`tag_3` FROM `image_tags` t
WHERE t.`type` = 1 AND t.`category` LIKE "00%" AND tag_1 = "0"
--- DESCRIBE OUTPUT
--- The used index thirdtag is just an index defined as (type, category, tag_1, tag_3)
--- The actual result is 201 rows
+----+-------------+-------+- -----------------------+----------+---------+------+---------+-------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+-----------------+----------+---------+------+---------+-------------------------------------------+
| 1 | SIMPLE | t | range | [... A LOT ...] | thirdtag | 31 | NULL | 1652861 | Using where; Using index; Using temporary |
+----+-------------+-------+-------+-----------------+----------+---------+------+---------+-------------------------------------------+
The only thing that's standing out is the enormous amount of rows involved. If you compare with the 2 fast queries I attached to the end of this question it is literally the only thing different (at least from the first one). So most probably that's the problem. But that's how the data is given to me so I need to work with that. I thought if involved in the index mysql could handle the data just fine.
Does anybody have a suggestion how to optimize the query? Any suggestions if i could use different indexes that suit more to the query?
For comparison these 2 similar queries work blazing fast
--- just a longer category string resulting in fewer results
SELECT DISTINCT t.`tag_3` FROM `image_tags` t
WHERE t.`type` = 1 AND t.`category` LIKE "0000%" AND tag_1 = "0"
--- and additional where clause
SELECT DISTINCT t.`tag_3` FROM `image_tags` t
WHERE t.`type` = 1 AND t.`category` LIKE "00%" AND tag_1 = "0" and tag_2 = ""
The table (it has a lot of indexes probably too long to paste).
+----------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| image | char(8) | NO | MUL | NULL | |
| category | varchar(6) | YES | MUL | NULL | |
| type | tinyint(1) | NO | MUL | NULL | |
| tag_1 | char(3) | NO | MUL | NULL | |
| tag_2 | char(3) | NO | MUL | NULL | |
| tag_3 | char(3) | NO | MUL | NULL | |
| tag_4 | char(3) | NO | MUL | NULL | |
| tag_5 | char(3) | NO | MUL | NULL | |
| tag_6 | char(3) | NO | MUL | NULL | |
+----------+------------------+------+-----+---------+----------------+

Answer 1

Please provide SHOW CREATE TABLE, it is more descriptive than DESCRIBE! In particular, I cannot see what indexes you have.
As My index cookbook explains, start the index with any fields that are '=', then you get one chance to add a 'range' comparison. Your category is a range, so
WHERE t.`type` = 1 AND t.`category` LIKE "00%" AND tag_1 = "0"
does not get past category in
INDEX(type, category, tag_1, tag_3)
For your 3 queries, these are the best indexes:
INDEX(type, tag_1, category)
INDEX(type, tag_1, category)
INDEX(type, tag_1, tag_2, category)
category should be last; the other columns can be in any order. Perhaps some one of your indexes handled the 3rd case?
it has a lot of indexes probably too long to paste
Probably most of them are unused. Keep in mind that INDEX(a) is unnecessary if you also have INDEX(a,b).