MySQL optimize select count distinct group by - mysql

Question

I have this query:
SELECT `variationID`, count(DISTINCT(`userID`))
FROM data WHERE `testID` = XXXX AND `visit` = 1 GROUP BY `variationID`
;
that takes a lot of time to query.How I can speed up the query.
select_type table type possible_keys key key_len ref rows
filtered Extra SIMPLE data
ref dc3_testIDPage,dc3_testIDvarIDPage,user_test_varID_url
dc3_testIDvarIDPage 8 const 33106102 100.00 Using where
This is the output of the create table:
CREATE TABLE `data` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`userID` bigint(17) NOT NULL,
`testID` bigint(20) NOT NULL,
`variationID` bigint(20) NOT NULL,
`url` bigint(20) NOT NULL,
`time` bigint(20) NOT NULL,
`visit` bigint(20) NOT NULL DEFAULT '1',
`isTestPage` tinyint(1) NOT NULL,
PRIMARY KEY (`id`,`testID`),
KEY `url` (`url`),
KEY `dc3_testIDPage` (`testID`,`url`),
KEY `dc3_testIDvarIDPage` (`testID`,`variationID`,`url`),
KEY `user_test_url` (`userID`,`testID`,`url`),
KEY `user_test_varID_url` (`userID`,`testID`,`variationID`,`url`)
) ENGINE=InnoDB AUTO_INCREMENT=67153224 DEFAULT CHARSET=latin1

Answer 1

The easiest thing you can do to speed up your query is to make sure you are not doing full table scans. All the columns in your where clause should appear in indexes. So in your case testID and visit should have indexes and even better you can create a single index with both testID and visit. If visit is a true/false boolean that won't help narrow the index search much but testID certainly will.
Create index documentation is here: http://dev.mysql.com/doc/refman/5.7/en/create-index.html
Based on the create table you have id and testID are in a single primary key. Add a new key or index that only has testID in it. That should help quite a bit. Since it looks like visit is not a boolean adding an index with both visit and testID will give you the best performance boost.