MySQL Index with ordering - mysql

Question

I have a table with 5 million rows. I didn't add my indexes here:
CREATE TABLE `my_table` (
`Id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`Title` CHAR(200) NULL DEFAULT NULL,
`ProjectId` INT(10) UNSIGNED NOT NULL,
`RoleId` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY (`Id`)
)
COLLATE='latin1_swedish_ci'
ENGINE=InnoDB;
When I run below SQL, it takes more than 1 minute.
SELECT *
FROM `my_table` t
WHERE
t.ProjectId IN (123, 456, 789) AND
t.RoleId IN (111, 222, 333)
ORDER BY Title DESC
LIMIT 25
Question is, how properly add indexes for the table. Can you give any solutions?
Explain for index "ProjectId" and "RoleId" is:
key = IndxProjectIdRoleId
ref = NULL,
rows: 32,463
Extra: Using where; Using filesort
Thanks for any suggestion.

Answer 1

You can try indexes on (ProjectId, RoleId, Title) and (RoleId, ProjectId, Title). They may not help much. The problem is that you have two inequalities in the where.
One of these is likely to be better than the current execution plan. However, it might not help so much.
MySQL actually has good documentation on multi-column indexes. You might want to review it.
A more complicated version of the query might work better:
(SELECT *
FROM `my_table` t
WHERE t.ProjectId = 123 AND t.RoleId = 111
ORDER BY Title DESC
LIMIT 25
) UNION ALL
(SELECT *
FROM `my_table` t
WHERE t.ProjectId = 123 AND t.RoleId = 456
ORDER BY Title DESC
LIMIT 25
)
UNION ALL
. . . -- The other 7 combinations
ORDER BY Title DESC
LIMIT 25;
This much longer version of the query can take advantage of either of the above indexes so each should be quite fast. In the end, the query has to sort up to 9 * 25 (225) records, and that should be pretty fast, even without an index.

Answer 2

I suggest a composite index
INDEX my_index_name (ProjectId,RoleId )
in your case ..
CREATE TABLE `my_table` (
`Id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`Title` CHAR(200) NULL DEFAULT NULL,
`ProjectId` INT(10) UNSIGNED NOT NULL,
`RoleId` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY (`Id`),
INDEX my_index_name (ProjectId,RoleId)
)
COLLATE='latin1_swedish_ci'
ENGINE=InnoDB;
eventually check if is more selective the inverse
INDEX my_index_name (RoleId, ProjectId)
And do the fact your table has only few column you can also try a complete indexed table
INDEX my_index_name (ProjectId,RoleId, Tile, id)
and select this way
SELECT Id, Title, ProjectId, RoleId
FROM `my_table` t
WHERE
t.ProjectId IN (123, 456, 789) AND
t.RoleId IN (111, 222, 333)
ORDER BY Title DESC
LIMIT 25;