MySQL - how to optimize query with order by - mysql

Question

I am trying to generate a list of the 5 most recent history items for for a collection of user tasks. If I remove the order by the execution drops from ~2 seconds to < 20msec.
Indexes are on
h.task_id
h.mod_date
i.task_id
i.user_id
This is the query
SELECT h.*
, i.task_id
, i.user_id
, i.name
, i.completed
FROM h
, i
WHERE i.task_id = h.task_id
AND i.user_id = 42
ORDER
BY h.mod_date DESC
LIMIT 5
Here is the explain:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE i ref PRIMARY,UserID UserID 4 const 3091 Using temporary; Using filesort
1 SIMPLE h ref TaskID TaskID 4 myDB.i.task_id 7
Here are the show create tables:
CREATE TABLE `h` (
`history_id` int(6) NOT NULL AUTO_INCREMENT,
`history_code` tinyint(4) NOT NULL DEFAULT '0',
`task_id` int(6) NOT NULL,
`mod_date` datetime NOT NULL,
`description` text NOT NULL,
PRIMARY KEY (`history_id`),
KEY `TaskID` (`task_id`),
KEY `historyCode` (`history_code`),
KEY `modDate` (`mod_date`)
) ENGINE=InnoDB AUTO_INCREMENT=185647 DEFAULT CHARSET=latin1
and
CREATE TABLE `i` (
`task_id` int(6) NOT NULL AUTO_INCREMENT,
`user_id` int(6) NOT NULL,
`name` varchar(60) NOT NULL,
`due_date` date DEFAULT NULL,
`create_date` date NOT NULL,
`completed` tinyint(1) NOT NULL DEFAULT '0',
`task_description` blob,
PRIMARY KEY (`task_id`),
KEY `name_2` (`name`),
KEY `UserID` (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=12085 DEFAULT CHARSET=latin1

Answer 1

INDEX(task_id, mod_date, history_id) -- in this order
Will be "covering" and the columns will be in the optimal order
Also, DROP
KEY `TaskID` (`task_id`)
So that the Optimizer won't be tempted to use it.

Answer 2

Try changing the index on h.task_id so it's this compound index.
CREATE OR REPLACE INDEX TaskID ON h(task_id, mod_date DESC);
This may (or may not) allow MySql to shortcut some or all the extra work in your ORDER BY ... LIMIT ... request. It's a notorious performance anti pattern, by the way, but sometimes necessary.
Edit the index didn't help. So let's try a so-called deferred join so we don't have to ORDER and then LIMIT all the data from your h table.
Start with this subquery. It retrieves only the primary key values for the rows involved in your results, and will generate just five rows.
SELECT h.history_id, i.task_id
FROM h
JOIN i ON h.task_id = i.task_id
WHERE i.user_id = 42
ORDER BY h.mod_date
LIMIT 5
Why this subquery? It handles the work-intensive ORDER BY ... LIMIT operation while manipulating only the primary keys and the date. It still must sort tons of rows only to discard all but five, but the rows it has to handle are much shorter. Because this subquery does the heavy work, you focus on optimizing it, rather than the whole query.
Keep the index I suggested above, because it covers the subquery for h.
Then, join it to the rest of your query like this. That way you'll only have to retrieve the expensive h.description column for the five rows you care about.
SELECT h.* , i.task_id, i.user_id , i.name, i.completed
FROM h
JOIN i ON i.task_id = h.task_id
JOIN (
SELECT h.history_id, i.task_id
FROM h
JOIN i ON h.task_id = i.task_id
WHERE i.user_id = 42
ORDER BY h.mod_date
LIMIT 5
) selected ON h.history_id = selected.history_id
AND i.task_id = selected.task_id
ORDER BY h.mod_date DESC
LIMIT 5