Very slow SQL query with only two left joins - mysql

Question

I am joining three tables 'customer', 'customer_address' and 'country' using left join because I'm allowing a customer to have one or none address.
At the moment I have 13k+ customers and the query takes about 40 sec. I tried inner join but in that case I'm not getting the customers with no address.
All columns in 'ON' are indexed but it doesn't make much of a difference.
Here is my query:
SELECT DISTINCT *,
CASE
WHEN customer_address.customerid is NULL THEN customer.customerid
ELSE customer_address.customerid
END as customerid,
CASE
WHEN address1 = '' THEN 'NA'
ELSE address1
END as address1
FROM customer
LEFT JOIN customer_address ON customer.customerid = customer_address.customerid
LEFT JOIN country ON country.id = customer_address.country
WHERE deleted='0'
ORDER BY customer.customerid
DESC
LIMIT 0, 10
Any help would be appreciated
EDIT:
Here is 'explain' for the three tables:
customer
Field Type Null Key Default Extra
customerid int(12) NO PRI NULL auto_increment
forename varchar(128) YES NULL
surname varchar(128) YES NULL
company varchar(64) YES NULL
tel varchar(32) YES NULL
tel2 varchar(32) YES NULL
fax varchar(32) YES NULL
mob varchar(32) YES NULL
email varchar(255) YES NULL
date_reg date YES NULL
last_update datetime YES NULL
deleted int NO
customer_address
Field Type Null Key Default Extra
addressid varchar(12) NO PRI
customerid varchar(12) YES MUL NULL
address1 varchar(128) YES NULL
address2 varchar(128) YES NULL
town varchar(128) YES NULL
county varchar(128) YES MUL NULL
postcode varchar(12) YES NULL
country int(12) YES NULL
address_date datetime YES NULL
isprimary int NO not
country
Field Type Null Key Default Extra
id int(12) NO PRI 0
country varchar(255) YES NULL
At the moment there are no deleted!='0'
EDIT 2:
Query Explain:
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
1 SIMPLE customer NULL ALL deleted NULL NULL NULL 13082 99.98 Using where; Using temporary; Using filesort
1 SIMPLE customer_address NULL ALL NULL NULL NULL NULL 9983 100.00 Using where; Using join buffer (Block Nested Loop)
1 SIMPLE country NULL eq_ref PRIMARY,id PRIMARY 4 db_name.customer_address.country 1 100.00 NULL
EDIT 3:
1 SIMPLE customer NULL index NULL customerid 4 NULL 1 10.00 Using where; Using temporary
1 SIMPLE customer_address NULL ALL NULL NULL NULL NULL 9983 100.00 Using where
1 SIMPLE country NULL eq_ref PRIMARY,id PRIMARY 4 db_name.customer_address.country 1 100.00 NULL

Answer 1

Well, you have to ALL type queries that do not use any indexes. One of them even has the dreaded filesort, which is a very expensive operation.
Add an index on customer_address.customerid field. This will be used to match the records from customer_address table to the main customer table.
List the columns you want to return from the query, do not use *. For example, I do not see why you need to return customerid from both the customer and the address tables.
Get rid of the 1st case statement. customer.customerid field will always be populated.
Add an index hint after the customer table to make mysql think about using the customerid index for sorting:
...
FROM customer FORCE INDEX index_name_forcustomerid_field
...
You may want to consider increasing join_buffer_size server variable, however, adding the index in the first place should help a lot.

Answer 2

you can try this. You do not need to use first CASE statement as You are never going to receive CustomerId as NULL. I have removed ORDER BY clause too as I am assuming it will improve query perfomance (CustomerId is a primary key which shows how records are physically arranged in database. The default arrangement order is Ascending.)
SELECT DISTINCT *, C.customerid as customerid,
CASE
WHEN customer_address.address1 = '' THEN 'NA'
ELSE customer_address.address1
END as address1 from (select *
FROM customer where deleted='0' order by customerid DESC) AS C
LEFT JOIN customer_address ON C.customerid = customer_address.customerid
LEFT JOIN country ON country.id = customer_address.country
LIMIT 0, 10