Rebuild MySQL query to stay below MAX_JOIN_SIZE rows - mysql

Question

I have a SQL query which fails (most of the times) because of too many joined rows. The error provided by MySQL is The SELECT would examine more than MAX_JOIN_SIZE rows; check your WHERE and use SET SQL_BIG_SELECTS=1 or SET MAX_JOIN_SIZE=# if the SELECT is okay. I know I can avoid the error by setting the mentioned variables SQL_BIG_SELECTS and MAX_JOIN_SIZE, but I feel like this isn't the right way and pushes the problem only a bit in the future, because the join count might grow in the future.
The facts: I have an event planning tool which assigns users (=workers) to certain tasks. The tables are users (userid,username) [ID and name], tasks (taskid,task,start,end) [ID, task name, start as timestamp, end as timestamp] and userassignment (id,userid,taskid,deleted) [ID, user assigned to a task, the task, is the assignment still valid).
The exact table definition is like this:
CREATE TABLE users (
userid INT NOT NULL AUTO_INCREMENT,
username VARCHAR(250),
PRIMARY KEY (userid)
);
CREATE TABLE tasks (
taskid INT NOT NULL AUTO_INCREMENT,
task VARCHAR(250),
start INT,
end INT,
PRIMARY KEY (taskid),
INDEX USING BTREE (start),
INDEX USING BTREE (end)
);
CREATE TABLE userassignment (
id INT NOT NULL AUTO_INCREMENT,
userid INT,
taskid INT,
deleted TINYINT,
PRIMARY KEY (id),
INDEX USING BTREE (userid),
INDEX USING BTREE (userid),
UNIQUE KEY `usertasks` ( `userid` , `taskid` )
);
I need to know, which users are assigned and on which main days of the event (day 1, day 2, day 3) they're assigned.
My query looks like this:
SELECT
u.userid,
u.username,
COUNT(ua.id) AS count_all,
dayone.c AS count_one,
daytwo.c AS count_two,
daythree.c AS count_three
FROM
users AS u
INNER JOIN
userassignment AS ua ON ua.userid = u.userid AND ua.deleted = 0
INNER JOIN
tasks AS t ON ua.taskid = t.taskid
LEFT JOIN (
SELECT
u.userid,
COUNT(ua.id) AS c
FROM
users AS u
INNER JOIN
userassignment AS ua ON
ua.userid = u.userid AND
ua.deleted = 0
INNER JOIN
tasks AS t ON
ua.taskid = t.taskid
WHERE
t.start > UNIX_TIMESTAMP("2014-08-01 00:00:00") AND
t.start < UNIX_TIMESTAMP("2014-08-02 00:00:00")
GROUP BY
u.userid
) AS dayone ON dayone.userid = u.userid
LEFT JOIN (
SELECT
u.userid,
COUNT(ua.id) AS c
FROM
users AS u
INNER JOIN
userassignment AS ua ON
ua.userid = u.userid AND
ua.deleted = 0
INNER JOIN
tasks AS t ON
ua.taskid = t.taskid
WHERE
t.start > UNIX_TIMESTAMP("2014-07-31 00:00:00") AND
t.start < UNIX_TIMESTAMP("2014-08-01 00:00:00")
GROUP BY
u.userid
) AS daytwo ON daytwo.userid = u.userid
LEFT JOIN (
SELECT
u.userid,
COUNT(ua.id) AS c
FROM
users AS u
INNER JOIN
userassignment AS ua ON
ua.userid = u.userid AND
ua.deleted = 0
INNER JOIN
tasks AS t ON
ua.taskid = t.taskid
WHERE
t.start > UNIX_TIMESTAMP("2014-08-02 00:00:00") AND
t.start < UNIX_TIMESTAMP("2014-08-04 00:00:00")
GROUP BY
u.userid
) AS daythree ON daythree.userid = u.userid
WHERE
t.start > UNIX_TIMESTAMP("2014-07-31 00:00:00") AND
t.start < UNIX_TIMESTAMP("2014-08-04 00:00:00")
GROUP BY
u.userid
ORDER BY
username ASC
First I select all users which have an assignment in one of the three days (there are about six time more users in the DB than assigned to a task), then I left join the assigned users of every of the three days.
So, is there a way to rebuild the query to join fewer rows? I only need to know, who is assigned on which day, not the number of assignments.
I already tried to UNION several queries but this was unsuccessful.
SQL Fiddle
An EXPLAIN of the real query (not in the SQL Fiddle) is:
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY t range PRIMARY,start start 5 NULL 120 100.00 Using where; Using index; Using temporary; Using filesort
1 PRIMARY ua ref usertasks,userid,taskid taskid 2 db1154575-helfer.t.id 2 100.00 Using where
1 PRIMARY u eq_ref userid userid 2 db1154575-helfer.ua.userid 1 100.00
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 152 100.00
1 PRIMARY <derived3> ALL NULL NULL NULL NULL 94 100.00
1 PRIMARY <derived4> ALL NULL NULL NULL NULL 147 100.00
4 DERIVED t range PRIMARY,start start 5 NULL 53 100.00 Using where; Using index; Using temporary; Using filesort
4 DERIVED ua ref usertasks,userid,taskid taskid 2 db1154575-helfer.t.id 2 100.00 Using where
4 DERIVED u eq_ref userid userid 2 db1154575-helfer.ua.userid 1 100.00 Using index
3 DERIVED t range PRIMARY,start start 5 NULL 21 100.00 Using where; Using index; Using temporary; Using filesort
3 DERIVED ua ref usertasks,userid,taskid taskid 2 db1154575-helfer.t.id 2 100.00 Using where
3 DERIVED u eq_ref userid userid 2 db1154575-helfer.ua.userid 1 100.00 Using index
2 DERIVED t range PRIMARY,start start 5 NULL 44 100.00 Using where; Using index; Using temporary; Using filesort
2 DERIVED ua ref usertasks,userid,taskid taskid 2 db1154575-helfer.t.id 2 100.00 Using where
2 DERIVED u eq_ref userid userid 2 db1154575-helfer.ua.userid 1 100.00 Using index

Answer 1

So, is all that really just a long-winded way of saying this...
SELECT u.*
, DATE(FROM_UNIXTIME(t.start)) dt
, COUNT(t.taskid) total
FROM users u
LEFT
JOIN userassignment ut
ON ut.userid = u.userid
AND ut.deleted = 0
LEFT
JOIN tasks t
ON t.taskid = ut.taskid
GROUP
BY u.userid
, DATE(FROM_UNIXTIME(t.start))
In the example above, you can change COUNT(t.taskid) to COUNT(CASE WHEN x = 'y' THEN z END) or SUM(CASE...

Answer 2

This should return the same result set:
SELECT u.userid, u.username,
COUNT(ua.id) AS count_all,
SUM(case when t.start > UNIX_TIMESTAMP('2014-08-01 00:00:00') AND
t.start < UNIX_TIMESTAMP('2014-08-02 00:00:00')
then 1 else 0
end) as count_one,
SUM(case when t.start > UNIX_TIMESTAMP('2014-07-31 00:00:00') AND
t.start < UNIX_TIMESTAMP('2014-08-01 00:00:00')
then 1 else 0
end) as count_two,
SUM(case when t.start > UNIX_TIMESTAMP('2014-08-02 00:00:00') AND
t.start < UNIX_TIMESTAMP('2014-08-04 00:00:00')
then 1 else 0
end) as count_three
FROM users u LEFT JOIN
userassignment ua
ON ua.userid = u.userid AND
ua.deleted = 0 LEFT JOIN
tasks t
ON ua.taskid = t.taskid
WHERE ua.deleted = 0 AND
t.start > UNIX_TIMESTAMP('2014-07-31 00:00:00') AND
t.start < UNIX_TIMESTAMP('2014-08-04 00:00:00')
GROUP BY u.userid
ORDER BY u.username;
Your formulation is a bit tricky. The outer joins are filter out any user whose assignments are always deleted, for instance. And the date periods are overlapping (I'm not sure if that is intentional, but it is how the query is structured).
Perhaps this simpler query will not exceed internal limits.