how to execute sql select query with minimum runtime

Question

I want to select from a table and then make select for each of them.

tables:

category
+====+=======+
| id | title |
+====+=======+
this table has list of category

email
+====+=======+==========+=============+
| id | eMail | domainId | elseColumns |
+====+=======+==========+=============+
this table has list of emails but domains are in another table

domain
+====+========+=============+
| id | domain | elseColumns |
+====+========+=============+
list of domains which used in email

subscriber_category
+========+============+
| userId | categoryId |
+========+============+
list of emails in categories

now the question is how can i list categories and count of emails in them with minimum runtime? my try is waiting 20sec for 200000 email and 20 category.

sql:

SELECT category.*,
(SELECT COUNT(DISTINCT subscriber_category.userId) FROM subscriber_category
    JOIN email ON email.id=subscriber_category.userId
    JOIN domain ON domain.id=email.domainId
WHERE subscriber_category.categoryId=category.id
    AND email.blackList=0
    AND domain.blackList=0
) AS qty
FROM category WHERE category.userId=1 ORDER BY category.title ASC

Answer 1

This is your query:

SELECT c.*,
       (SELECT COUNT(DISTINCT sc.userId)
        FROM subscriber_category sc JOIN
             email e
             ON e.id = sc.userId JOIN
             domain d
             ON d.id = e.domainId
       WHERE sc.categoryId = c.id AND e.blackList = 0 AND d.blackList = 0
      ) AS qty
FROM category c
WHERE c.userId = 1
ORDER BY c.title ASC;

The structure is quite reasonable and indexes should help performance. The first index is on category(userId, title, id) . This index should be used for the WHERE clause, the ORDER BY , and the correlated subquery.

Next, I assume that you have indexes on the id columns in email and domain . You could make these slightly more applicable to the query if you include the blacklist flag as a second column in the index. More importantly, you want an index on subscriber_category(categoryId, userId) . I would also recommend removing the count(distinct) if it is not necessary.