MySQL COUNT of multiple left joins - optomization - mysql

Question

I have a query that is getting counts from multiple tables by using a LEFT JOIN and subqueries. The idea is to get a count various activites a member has participated in.
The schema looks like this:
member
PK member_id
table1
PK tbl1_id
FK member_id
table2
PK tbl2_id
FK member_id
table3
PK tbl3_id
FK member_id
My query looks like this:
SELECT t1.num1,t2.num2,t3.num3
FROM member m
LEFT JOIN
(
SELECT member_id,COUNT(*) as num1
FROM table1
GROUP BY member_id
) t1 ON t1.member_id = m.member_id
LEFT JOIN
(
SELECT member_id,COUNT(*) as num2
FROM table2
GROUP BY member_id
) t2 ON t2.member_id = m.member_id
LEFT JOIN
(
SELECT member_id,COUNT(*) as num3
FROM table3
GROUP BY member_id
) t3 ON t3.member_id = m.member_id
WHERE m.member_id = 27
Where 27 is a test id. The actual query joins more than three tables and the query is run multiple times with the member_id being changed. The problem is this query runs pretty slow. I get the info I need but I am wondering if anyone could suggest a way to optimize this. Any advice is very much appreciated. Thanks much.

Answer 1

You should refactor your query. You can do this by reordering the way the query collects the data. How?
Apply the WHERE clause first
Apply JOINs last
Here is your original query:
SELECT t1.num1,t2.num2,t3.num3
FROM member m
LEFT JOIN
(
SELECT member_id,COUNT(*) as num1
FROM table1
GROUP BY member_id
) t1 ON t1.member_id = m.member_id
LEFT JOIN
(
SELECT member_id,COUNT(*) as num2
FROM table2
GROUP BY member_id
) t2 ON t2.member_id = m.member_id
LEFT JOIN
(
SELECT member_id,COUNT(*) as num3
FROM table3
GROUP BY member_id
) t3 ON t3.member_id = m.member_id
WHERE m.member_id = 27
Here is you new query
SELECT
IFNULL(t1.num1,0) num1,
IFNULL(t1.num2,0) num2,
IFNULL(t1.num3,0) num3
FROM
(
SELECT * FROM member m
WHERE member_id = 27
)
LEFT JOIN
(
SELECT member_id,COUNT(*) as num1
FROM table1
WHERE member_id = 27
GROUP BY member_id
) t1 ON t1.member_id = m.member_id
LEFT JOIN
(
SELECT member_id,COUNT(*) as num2
FROM table2
WHERE member_id = 27
GROUP BY member_id
) t2 ON t2.member_id = m.member_id
LEFT JOIN
(
SELECT member_id,COUNT(*) as num3
FROM table3
WHERE member_id = 27
GROUP BY member_id
) t3 ON t3.member_id = m.member_id
;
BTW I changed member m into SELECT * FROM member m WHERE member_id = 27 in case you need any information about member 27. I also added the IFNULL function to each result to produce 0 in case count is NULL.
You need to make absolutely sure
member_id is the primary key of the member table
member_id is indexed in table1, table2, and table3
Give it a Try !!!

Answer 2

Without knowing your schema and what you've done for indexes, one POSSIBLE way to make this faster is:
SELECT (select ifnull(count(*),0) from table1 where table1.member_id = m.id) as num1,
(select ifnull(count(*),0) from table2 where table2.member_id = m.id) as num2,
(select ifnull(count(*),0) from table3 where table3.member_id = m.id) as num3
from member m
WHERE m.member_id = 27
Now, this is a slightly risky recommendation, simply because I don't know anything about your DB or what else is running, or where the bottlenecks are.
In general, it would be a good idea to post an explain plan with your query to get a better answer.

Answer 3

SELECT num1, num2, count(*) as num3
FROM (
SELECT member_id, num1, count(*) as num2
FROM (
SELECT member_id, count(*) as num1
FROM member
LEFT JOIN table1 USING (member_id)
WHERE member_id = 27) as m1
LEFT JOIN table2 USING (member_id)) as m2
LEFT JOIN table3 USING (member_id);