Force query optimizer to use Primary key - mysql

Question

I'm running a query that spans 3 tables, none of which have > 55K rows. This query is taking 20+ seconds to run, which seems excessive:
SELECT
`cp`.`author`,
`cc`.`contents`
FROM
`challenge_properties` as `cp`,
`challenges` as `c`,
`challenge_contents` as `cc`
WHERE
`cp`.`followup_id` = `c`.`latest_followup` AND
`cp`.`status` = 'new' AND
`c`.`id` = `cp`.`challenge_id` AND
`c`.`id` = `cc`.`challenge_id`
This is the result of EXPLAINing that query:
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: c
type: ALL
possible_keys: PRIMARY,latest_followup_index
key: NULL
key_len: NULL
ref: NULL
rows: 13817
Extra:
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: cc
type: ref
possible_keys: challenge_id
key: challenge_id
key_len: 5
ref: cts.c.id
rows: 1
Extra: Using where
*************************** 3. row ***************************
id: 1
select_type: SIMPLE
table: cp
type: ref
possible_keys: challenge_id,followup_id
key: followup_id
key_len: 5
ref: cts.c.latest_followup
rows: 1
Extra: Using where
As you can see, the first table, challenges has a primary key, but it's not being used. I've tried adding the FORCE KEY(PRIMARY) clause to the challenges table declaration, but it's still not used.
What can I do to speed up this query? Thanks.

Answer 1

Your query is selecting ALL records from the challenges table — therefore there is no need to use any index on the records from that table. Basically MySQL is selecting every record in challenges, then finding matching records in the other two tables.
Couldn't you just leave out the challenges table all together? You're not selecting data from that table, and the only time that table would limit the data selected would be if your other tables had invalid challenge_ids, which foreign keys can take care of...
SELECT
`cp`.`author`,
`cc`.`contents`
FROM
`challenge_properties` as `cp`,
`challenge_contents` as `cc`
WHERE
`cp`.`status` = 'new' AND
`cp`.`challenge_id` = `cc`.`challenge_id`
EDIT: You say you can't remove the challenges table from the query... I would try specifying your JOIN conditions in the JOIN clause instead of the WHERE:
SELECT
`cp`.`author`,
`cc`.`contents`
FROM `challenge_properties` as `cp`
JOIN `challenges` as `c`
ON `cp`.`challenge_id` = `c`.`id`
AND `cp`.`followup_id` = `c`.`latest_followup`
JOIN `challenge_contents` as `cc`
ON `cc`.`challenge_id` = `c`.`id`
WHERE `cp`.`status` = 'new'
The query optimizer might already do this for you, but it doesn't hurt to try it, and I think it's easier to see how the joins are happening with this syntax.
You could also try adding another index to challenge_properties on ( challenge_id, followup_id ), and another to challenges on ( challenge_id, latest_followup ) — the complex key might help MySQL work quicker. But it's also possible that the problem might be outside your query... usually when you EXPLAIN and see only one table with big numbers in the rows column, your query is pretty well optimized. MySQL is only looking at one row in challenge_properties and one row in challenge_contents, and scanning every row in challenges to find a match.
EDIT 2:
Unfortunately, I'm not sure what else can be done to optimize this query. You can get slightly more performance if the indexes used (cc.challenge_id and cp.followup_id) are UNIQUE NOT NULL indexes, and you should get better performance with a complex index for cp on (cp.challenge_id, cp.followup_id). This would turn those type: ref into type: eq_ref, which is slightly better. But that's about it... do you not have problems with any other queries? Your query should theoretically return 13817 rows... is the amount of data possibly the problem? Does it speed up significantly if you just select COUNT(*) instead of returning all the rows?