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 !!!