MYSQL Indexes when looking up on alternating sets of columns


I have a table that looks like:

CREATE TABLE `connections` (
`connection_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`service_id` smallint(5) unsigned DEFAULT NULL,
`parent_id` bigint(20) unsigned DEFAULT NULL,
`child_id` bigint(20) unsigned DEFAULT NULL,
PRIMARY KEY (`connection_id`),
UNIQUE KEY `primary_child` (`parent_id`,`child_id`),
UNIQUE KEY `child_primary` (`child_id`,`parent_id`)

I need to do lookups on child_id and get information about parent_id and vice versa.


SELECT COUNT(`parent_id`) FROM `connections` WHERE `child_id` = 'x'


SELECT COUNT(`child_id`) FORM `connections` WHERE `parent_id` = 'x'

And some more advanced queries like:

FROM `connections` `c1`
INNER JOIN `connections` `c2`
ON `c1`.`parent_id` = `c2`.`parent_id`
WHERE `c1`.`child_id` = 'x'
AND `c2`.`child_id` = 'y'

I'm looking to get a better idea of the most efficient index structure to get quick responses from all the above query examples.

It should be noted that the child/parent pairs will be unique.

EDIT: This table currently holds 40M rows, likely to be much larger in the near future.


Best Answer

You already have indices on both colums with the unique constraint. No need for new ones.

I hope you have a good reason avoiding a foreign key. I hope you pay attention to keep your data consistent...

btw: parent_id in your join query is ambiguous.