MySQL query with JOIN and GROUP BY optimization. Is it possible? - mysql

Question

I have two tables: gpnxuser and key_value
mysql> describe gpnxuser;
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| id | bigint(20) | NO | PRI | NULL | auto_increment |
| version | bigint(20) | NO | | NULL | |
| email | varchar(255) | YES | | NULL | |
| uuid | varchar(255) | NO | MUL | NULL | |
| partner_id | bigint(20) | NO | MUL | NULL | |
| password | varchar(255) | YES | | NULL | |
| date_created | datetime | YES | | NULL | |
| last_updated | datetime | YES | | NULL | |
+--------------+--------------+------+-----+---------+----------------+
and
mysql> describe key_value;
+----------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------+--------------+------+-----+---------+----------------+
| id | bigint(20) | NO | PRI | NULL | auto_increment |
| version | bigint(20) | NO | | NULL | |
| date_created | datetime | YES | | NULL | |
| last_updated | datetime | YES | | NULL | |
| upkey | varchar(255) | NO | MUL | NULL | |
| user_id | bigint(20) | YES | MUL | NULL | |
| security_level | int(11) | NO | | NULL | |
+----------------+--------------+------+-----+---------+----------------+
key_value.user_id is FK that references gpnxuser.id. I also have an index in gpnxuser.partner_id which is a FK that references a table called "partner" (which, I think, does not matter much to this question).
For partner_id = 64, I have 500K rows in gpnxuser which have relationship with approximatelly 6M rows in key_value.
I wanted to have a query that returned all distinct 'key_value.upkey' for user´s belonging to a given partner. I did something like this:
select upkey from gpnxuser join key_value on gpnxuser.id=key_value.user_id where partner_id=64 group by upkey;
which takes forever to run. The explain for the query looks like:
mysql> explain select upkey from gpnxuser join key_value on gpnxuser.id=key_value.user_id where partner_id=64 group by upkey;
+----+-------------+-----------+------+----------------------------+--------------------+---------+-----------------------------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+----------------------------+--------------------+---------+-----------------------------+--------+----------------------------------------------+
| 1 | SIMPLE | gpnxuser | ref | PRIMARY,FKB2D9FEBE725C505E | FKB2D9FEBE725C505E | 8 | const | 259640 | Using index; Using temporary; Using filesort |
| 1 | SIMPLE | key_value | ref | FK9E0C0F912D11F5A9 | FK9E0C0F912D11F5A9 | 9 | gpnx_finance_db.gpnxuser.id | 14 | Using where |
+----+-------------+-----------+------+----------------------------+--------------------+---------+-----------------------------+--------+----------------------------------------------+
My question is: is there a query that can run fast and obtain the result that I want?

Answer 1

what you need to do is utilize EXISTS statement: This will cause only partial table scan until a match found and not more.
select upkey from (select distinct upkey from key_value) upk
where EXISTS
(select 1 from gpnxuser u, key_value kv
where u.id=kv.user_id and partner_id=1 and kv.upkey = upk.upkey)
NB. In the original query, group by is misused: distinct looks better there.
select DISTINCT upkey from gpnxuser join key_value on
gpnxuser.id=key_value.user_id where partner_id=1

Answer 2

I would look into partitioning your key_value table on user_id, if you typically run queries based on this column.
http://dev.mysql.com/doc/refman/5.1/en/partitioning.html