How Can I improve this MySQL query? - mysql

Question

I am trying to improve the performance of this query as it is taking 3-4 seconds to execute.
Here is the query
SELECT SQL_NO_CACHE
ac.account_id,
ac.account_name,
cl.name AS client_name,
IFNULL(cn.contact_number, "") AS Phone
FROM accounts AS ac
STRAIGHT_JOIN clients AS cl ON cl.client_id = ac.client_id
LEFT JOIN (
SELECT bc.contact_number, bc.account_id
FROM contact_numbers AS bc
INNER JOIN (
SELECT account_id, MAX(number_id) AS number_id
FROM contact_numbers
WHERE status = 1 AND contact_type != "Fax" AND contact_link = "Account"
GROUP BY account_id
) AS bb ON bb.number_id = bc.number_id
) AS cn ON ac.account_id = cn.account_id
WHERE ac.status = 1
ORDER BY ac.account_name
LIMIT 0, 100
the client table contains about 10 rows that's why I have straight join. The account table contains 350K records. The contact_numbers contains about 500k records
I believe the problem here is the left Join and also the ORDER BY but I am not sure how to work around it. Also I am using SQL_NO_CACHE because the accounts, contact_numbers tables are being updated at a fast rate.
What else can I do to improve performance of this query?
this is a screenshot of the explain on this query
I am using MySQL 5.6.13
I Set sort_buffer_size=1M
My server has 32GB of RAM

Answer 1

The below should make the outer query run without requiring a filesort.
CREATE INDEX ac_status_acctname ON accounts (status, account_name);
The below should make the inner query Using index, and help it to do the GROUP by without using a temp table.
CREATE INDEX cn_max ON contact_numbers (account_id, status, contact_link,
contact_type, number_id);
You need to join on both account_id and number_id to get the greatest entry per account. The way you have it now, you just get any account that happens to have the same number_id, which is probably not what you intended, and it could be what's generating too many rows for the subquery result set.
bc INNER JOIN ... bb ON bb.account_id = bc.account_id AND bb.number_id = bc.number_id
You can also write the same join condition as:
bc INNER JOIN ... bb USING (account_id, number_id)

Answer 2

I would actualy rewrite the query. You currently select a lot of data you do not need and discard. I would minimize the amount of the fetched data.
It seems you basically select something for each account with a certain status and take only 100 of them. So I would put this in a subquery:
SELECT
account_id,
account_name,
c.name AS client_name,
IFNULL(contact_number, '') as Phone
FROM (
SELECT
account_id,
MAX(number_id) as number_id
FROM (
SELECT account_id
FROM accounts
WHERE status = 1 -- other conditions on accounts go here
ORDER BY account_name
LIMIT 0, 100) as a
LEFT JOIN contact_numbers n
ON a.coount_id = n.account_id
AND n.status = 1
AND contact_type != "Fax"
AND contact_link = "Account"
GROUP BY account_id) an
LEFT JOIN contact_numbers USING (account_id, number_id)
JOIN accounts a USING (account_id)
JOIN clients c USING (client_id);
You will need (status, account_name) index for accounts table (for the query with client_id = 4 (status, client_id, account_name) as well) and an index on account_id in contact_numbers. This should suffice.