MYSQL order large database by decimal 10,10 - mysql

Question

I have about 25 million rows containing 0.183463545, 0.183423545, 0.183443545, 0.183443445, 0.183447545 and so on.
I need to order these however it currently takes around 20 seconds. Any way to speed it up? AFAIK, I have my index put it place correctly.
Thank you!
SELECT `a`.`float_val`,
`a`.`num_id`,
`b`.`userID`,
`c`.`img`,
`c`.`username`,
`b`.`img`,
`d`.`exterior`
FROM `a`
INNER JOIN `b` ON `b`.`num_id` = `a`.`num_id`
INNER JOIN `d` ON `d`.`id` = `b`.`item`
INNER JOIN `c` ON `c`.`userID` = `b`.`userID`
WHERE `float_val` IS NOT NULL
AND `float_val` BETWEEN 0 AND 1
AND `username` = 'ABC'
ORDER BY `float_val` LIMIT 100
Index is on float_val, num_id, userID
CREATE TABLE `float` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`num_id` bigint(11) DEFAULT NULL,
`float_val` decimal(10,10) DEFAULT NULL,
`userID` char(17) DEFAULT NULL,
`last_checked` datetime DEFAULT NULL,
`index10` smallint(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `floatID` (`num_id`),
UNIQUE KEY `num_id` (`num_id`,`userID`),
KEY `userID` (`userID`),
KEY `float_val` (`float_val`),
KEY `last_checked` (`last_checked`),
KEY `index10` (`index10`)
) ENGINE=InnoDB AUTO_INCREMENT=25750916 DEFAULT CHARSET=latin1;

Answer 1

Edited to reflect table definitions shown.
Start by doing a query to obtain the lowest float_val rows from table a.
SELECT a.id
FROM a
INNER JOIN b ON b.num_id = a.num_id
INNER JOIN d ON d.id = b.item
INNER JOIN c ON c.userID = b.userID
WHERE c.username = 'ABC'
AND a.float_val BETWEEN 0.0 AND 1.0
ORDER BY a.float_val
LIMIT 100
If you have an index on a(float_val,num_id), and another on c.username this will be reasonably fast. It will spit out the id values for the rows of a that are candidates for your query. (If you're using MyISAM, you need an index on a(float_val, num_id, id). By the way, the BETWEEN clause also implies IS NOT NULL.
Then use that as a subquery to complete your query, as follows.
SELECT a.float_val,
a.num_id,
b.userID,
c.img,
c.username,
b.img,
d.exterior
FROM a
INNER JOIN (
SELECT a.id
FROM a
INNER JOIN b ON b.num_id = a.num_id
INNER JOIN d ON d.id = b.item
INNER JOIN c ON c.userID = b.userID
WHERE c.username = 'ABC'
AND a.float_val BETWEEN 0.0 AND 1.0
ORDER BY a.float_val
LIMIT 100
) q ON a.ID = q.id
INNER JOIN b ON b.num_id = a.num_id
INNER JOIN d ON d.id = b.item
INNER JOIN c ON c.userID = b.userID
WHERE c.username = 'ABC'
AND a.float_val BETWEEN 0.0 AND 1.0
ORDER BY a.float_val LIMIT 100
This kind of query contains a deferred join. That dramatically reduces the number of rows of the full query that need to be subjected to ORDER BY ... LIMIT. Without the deferred join, your original query sorts an enormous mess of quite long rows just to discard all but the first hundred of them. That's why it takes so long.
This should help. The next optimization step is to look at the EXPLAIN output from this query and the exact definitions of your tables.
Pro tip: In queries of this complexity, always qualify column names with table names or aliases. That is, use a.float_val throughout, rather than just float_val. This is a kindness to the next person to look at the query.