How do I optimize my query in MySQL? - sql

Question

I need to improve my query, specially the execution time.This is my query:
SELECT SQL_CALC_FOUND_ROWS p.*,v.type,v.idName,v.name as etapaName,m.name AS manager,
c.name AS CLIENT,
(SELECT SEC_TO_TIME(SUM(TIME_TO_SEC(duration)))
FROM activities a
WHERE a.projectid = p.projectid) AS worked,
(SELECT SUM(TIME_TO_SEC(duration))
FROM activities a
WHERE a.projectid = p.projectid) AS worked_seconds,
(SELECT SUM(TIME_TO_SEC(remain_time))
FROM tasks t
WHERE t.projectid = p.projectid) AS remain_time
FROM projects p
INNER JOIN users m
ON p.managerid = m.userid
INNER JOIN clients c
ON p.clientid = c.clientid
INNER JOIN `values` v
ON p.etapa = v.id
WHERE 1 = 1
ORDER BY idName
ASC
The execution time of this is aprox. 5 sec. If i remove this part: (SELECT SUM(TIME_TO_SEC(remain_time)) FROM tasks t WHERE t.projectid = p.projectid) AS remain_time
the execution time is reduced to 0.3 sec. Is there a way to get the values of the remain_time in order to reduce the exec.time ?
The SQL is invoked from PHP (if this is relevant to any proposed solution).

Answer 1

It sounds like you need an index on tasks.
Try adding this one:
create index idx_tasks_projectid_remaintime on tasks(projectid, remain_time);
The correlated subquery should just use the index and go much faster.

Answer 2

Optimizing the query as it is written would give significant performance benefits (see below). But the FIRST QUESTION TO ASK when approaching any optimization is whether you really need to see all the data - there is no filtering of the resultset implemented here. This is a HUGE impact on how you optimize a query.
Adding an index on the query above will only help if the optimizer is opening a new cursor on the tasks table for every row returned by the main query. In the absence of any filtering, it will be much faster to do a full table scan of the tasks table.
SELECT ilv.*, remaining.rtime
FROM (
SELECT p.*,v.type, v.idName, v.name as etapaName,
m.name AS manager, c.name AS CLIENT,
SEC_TO_TIME(asbq.worked) AS worked, asbq.worked AS seconds_worked,
FROM projects p
INNER JOIN users m
ON p.managerid = m.userid
INNER JOIN clients c
ON p.clientid = c.clientid
INNER JOIN `values` v
ON p.etapa = v.id
LEFT JOIN (
SELECT a.projectid, SUM(TIME_TO_SEC(duration)) AS worked
FROM activities a
GROUP BY a.projectid
) asbq
ON asbq.projectid=p.projectid
) ilv
LEFT JOIN (
(SELECT t.project_id, SUM(TIME_TO_SEC(remain_time)) as rtime
FROM tasks t
GROUP BY t.projectid) remaining
ON ilv.projectid=remaining.projectid