MySQL左连接查询太慢

Question

I'm using this SQL query to join two tables and it takes 10 seconds even with a limit of 10. I've checked if the tables are indexed and they are so I'm out of ideas. Any help will be appreciated.

SELECT * FROM client_registration_request
LEFT JOIN customers ON (client_registration_request.customer_reference=customers.reference) 
LEFT JOIN region_info ON ( customers.country = region_info.id ) 
WHERE client_registration_request.client = 23
LIMIT 10

Here are the results from explain select

client_registration_request index

customers index

and region_info index

Answer 1

always make sure the fields you are using for your JOIN - conditions are indexed:

ALTER TABLE `client_registration_request` ADD INDEX `customer_reference` (`customer_reference`);
ALTER TABLE `customers` ADD INDEX `reference` (`reference`);

.. and so on.

update: your EXPLAIN shows that your indexed keys customers table are not used. so try to re-index them:

ALTER TABLE `customers` ADD INDEX `reference` (`reference`);
ALTER TABLE `customers` ADD INDEX `country` (`country`);

Answer 2

The way you have written the query is limiting and filtering the results after the two left join have performed, that's why it's taking the same time to query.

To make it quicker you need to limit and filter it as soon as possible, in your case you want to limit and filter it before any join.

As you are using LEFT JOINS, you can limit and filter the first table and still get the same result. Try this:

SELECT * FROM 
(SELECT * FROM client_registration_request WHERE client_registration_request.client = 23 LIMIT 10) client_registration_request_TMP
LEFT JOIN customers ON (client_registration_request_TMP.customer_reference=customers.reference) 
LEFT JOIN region_info ON ( customers.country = region_info.id )

Answer 3

Here is your query, using table aliases (so I find it easier to follow):

SELECT *
FROM client_registration_request crr LEFT JOIN
     customers c
     ON crr.customer_reference = c.reference LEFT JOIN
     region_info r
     ON c.country = r.id
WHERE crr.client = 23
LIMIT 10

The most important index is on on client_registration_request(client) . But, I would include the other join columns in this. So, the first index is client_registration_request(client, reference, country) . You can create this as:

create index idx_client_registration_request_3 on client_registration_request(client, reference, country);

I assume that the keys in the other tables are already primary keys or indexed. If not, you should build indexes on them too customer(reference) and region_info(country) .

Using a LIMIT without an ORDER BY is suspicious. You will be getting an arbitrary set of matching rows when you run the query -- you cannot guarantee that two runs would return the same set.