mysql query optimization - join and orderby

Question

I need a little help by one mysql query optimization. It is a simple query but anything is not right and I can't found it :-(

I have 2 tables: products (> 40000 Rows) and product_tags (> 5 mil)

There is a relation betweet the tables 1 -> N . Every prdoduct can have many tags in the table product tags.

I have this simple Query:

EXPLAIN SELECT t.product_id, kwt.tag_id
FROM products AS t, product_tags AS kwt
WHERE 1
AND t.product_id = kwt.product_id
AND kwt.tag_id =11
ORDER BY t.order_date 

wchich returns 55 results.

First Situation: if I have this table structure of the tables:

CREATE TABLE IF NOT EXISTS `products` (
  `product_id` int(10) unsigned NOT NULL auto_increment,
  `product_source_id` smallint(5) unsigned NOT NULL,
  `order_date` int(10) unsigned NOT NULL,
  PRIMARY KEY  (`product_id`),
  KEY `order_date` (`order_date`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 ;


CREATE TABLE IF NOT EXISTS `product_tags` (
  `product_tag_id` int(10) unsigned NOT NULL auto_increment,
  `tag_id` int(10) unsigned NOT NULL,
  `product_id` int(11) NOT NULL,
  PRIMARY KEY  (`product_tag_id`),
  KEY `product_id` (`product_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8  ;

Then the Explain of the query is this:

+----+-------------+-------+-------+---------------+------------+---------+---------------------------+-------+-------------+
| id | select_type | table | type  | possible_keys | key        | key_len | ref                       | rows  | Extra       |
+----+-------------+-------+-------+---------------+------------+---------+---------------------------+-------+-------------+
|  1 | SIMPLE      | t     | index | PRIMARY       | order_date | 4       | NULL                      | 45392 | Using index | 
|  1 | SIMPLE      | kwt   | ref   | product_id    | product_id | 4       | t.product_id          |     3 | Using where | 
+----+-------------+-------+-------+---------------+------------+---------+---------------------------+-------+-------------+

It is getting all the rows from table products, but there is nothing with temporary table.

Second Situation: If I add an index for the field "tag_id" in product_tags, then the picture is different:

+----+-------------+-------+--------+-------------------+---------+---------+-----------------------------+------+---------------------------------+
| id | select_type | table | type   | possible_keys     | key     | key_len | ref                         | rows | Extra                           |
+----+-------------+-------+--------+-------------------+---------+---------+-----------------------------+------+---------------------------------+
|  1 | SIMPLE      | kwt   | ref    | product_id,tag_id | tag_id  | 4       | const                       |   55 | Using temporary; Using filesort | 
|  1 | SIMPLE      | t     | eq_ref | PRIMARY           | PRIMARY | 4       | kwt.product_id              |    1 | Using where                     | 
+----+-------------+-------+--------+-------------------+---------+---------+-----------------------------+------+---------------------------------+

Now it selects only 55 rows, what is right, but the query is havy :(

Where is my mistake here ?

Thanks Nik

Answer 1

this is what i would do:

Read these resources

• http://dev.mysql.com/doc/refman/5.0/en/innodb-index-types.html

• http://www.xaprb.com/blog/2006/07/04/how-to-exploit-mysql-index-optimizations/

Redesign your system to take advantage of a clustered primary key

Simplified schema:

drop table if exists products;
create table products
(
prod_id int unsigned not null auto_increment primary key,
name varchar(255) not null unique
)
engine = innodb; 

drop table if exists tags;
create table tags
(
tag_id mediumint unsigned not null auto_increment primary key,
name varchar(255) not null unique
)
engine = innodb; 

drop table if exists product_tags;
create table product_tags
(
tag_id mediumint unsigned not null,
prod_id int unsigned not null,
created_date date not null,
primary key (tag_id, prod_id), -- note the clustered composite index and the order !!
key (prod_id)
)
engine = innodb;

Test the schema

select
 pt.tag_id,
 pt.prod_id
from
 product_tags pt
inner join products p on pt.prod_id = p.prod_id
where
 pt.tag_id = 11
order by
 pt.created_date
limit 10;

I may even change the product_tags PK to primary key (tag_id, prod_id, created_date) but it all depends on the typical queries you run. You could ofc, just create a non clustered secondary index on created date if you think that's gonna boost performance.

Hope this helps :)