Optimizing MySQL query to avoid scanning a lot of rows - mysql

Question

I am running an application that is using tables similar to the below tables. There are one tables for articles and there is another table for tags. I want to get the latest 30 articles for a specific tag order by article id. for example "acer", the below query will do the job but it is not indexed correctly because it will scan a lot of rows if there are a lot of articles related to a specific tag. How to run a query to get the same result without scanning a large number of rows?
EXPLAIN SELECT title
FROM tag, article
WHERE tag = 'acer'
AND tag.article_id = article.id
ORDER BY tag.article_id DESC
LIMIT 0 , 30
Output
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE tag ref tag tag 92 const 220439 Using where; Using index
1 SIMPLE article eq_ref PRIMARY PRIMARY 4 testdb.tag.article_id 1
The flollowing is the tables and sample data:
CREATE TABLE `article` (
`id` int(11) NOT NULL auto_increment,
`title` varchar(60) NOT NULL,
`time_stamp` int(11) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1000001 ;
--
-- Dumping data for table `article`
--
INSERT INTO `article` VALUES (1, 'Saudi Apple type D', 1313390211);
INSERT INTO `article` VALUES (2, 'Japan Apple type A', 1313420771);
INSERT INTO `article` VALUES (3, 'UAE Samsung type B', 1313423082);
INSERT INTO `article` VALUES (4, 'UAE Apple type H', 1313417337);
INSERT INTO `article` VALUES (5, 'Japan Samsung type D', 1313398875);
INSERT INTO `article` VALUES (6, 'UK Acer type B', 1313387888);
INSERT INTO `article` VALUES (7, 'Saudi Sony type D', 1313429416);
INSERT INTO `article` VALUES (8, 'UK Apple type B', 1313394549);
INSERT INTO `article` VALUES (9, 'Japan HP type A', 1313427730);
INSERT INTO `article` VALUES (10, 'Japan Acer type C', 1313400046);
CREATE TABLE `tag` (
`tag` varchar(30) NOT NULL,
`article_id` int(11) NOT NULL,
UNIQUE KEY `tag` (`tag`,`article_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
--
-- Dumping data for table `tag`
--
INSERT INTO `tag` VALUES ('Samsung', 1);
INSERT INTO `tag` VALUES ('Acer', 2);
INSERT INTO `tag` VALUES ('Sony', 3);
INSERT INTO `tag` VALUES ('Apple', 4);
INSERT INTO `tag` VALUES ('Acer', 5);
INSERT INTO `tag` VALUES ('HP', 6);
INSERT INTO `tag` VALUES ('Acer', 7);
INSERT INTO `tag` VALUES ('Sony', 7);
INSERT INTO `tag` VALUES ('Acer', 7);
INSERT INTO `tag` VALUES ('Samsung', 9);

Answer 1

What makes you think the query will examine a large number of rows?
The query will scan exactly 30 records using the UNIQUE index on tag (tag, article_id), join the article to each record on PRIMARY KEY and stop.
This is exactly what your plan says.
I just made this test script:
CREATE TABLE `article` (
`id` int(11) NOT NULL auto_increment,
`title` varchar(60) NOT NULL,
`time_stamp` int(11) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1000001 ;
CREATE TABLE `tag` (
`tag` varchar(30) NOT NULL,
`article_id` int(11) NOT NULL,
UNIQUE KEY `tag` (`tag`,`article_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
INSERT
INTO article
SELECT id, CONCAT('Article ', id), UNIX_TIMESTAMP('2011-08-17' - INTERVAL id SECOND)
FROM t_source;
INSERT
INTO tag
SELECT CASE fld WHEN 1 THEN CONCAT('tag', (id - 1) div 10 + 1) ELSE tag END AS tag, id
FROM (
SELECT tag,
id,
FIELD(tag, 'Other', 'Acer', 'Sony', 'HP', 'Dell') AS fld,
RAND(20110817) AS rnd
FROM (
SELECT 'Other' AS tag
UNION ALL
SELECT 'Acer' AS tag
UNION ALL
SELECT 'Sony' AS tag
UNION ALL
SELECT 'HP' AS tag
UNION ALL
SELECT 'Dell' AS tag
) t
JOIN t_source
) q
WHERE POWER(3, -fld) > rnd;
, where t_source is a table with 1M records in it, and run your query:
SELECT *
FROM tag t
JOIN article a
ON a.id = t.article_id
WHERE t.tag = 'acer'
ORDER BY
t.article_id DESC
LIMIT 30;
It was instant.

Answer 2

try ANSI join syntax:
SELECT title
FROM tag t
INNER JOIN article a
ON t.article_id = a.id
WHERE
t.tag = 'acer'
ORDER BY
tag.article_id DESC
LIMIT 0 , 30
then put an index on tag.tag. Assuming you have enough selectivity on that table, and article.id is a primary key, that should be pretty zippy.

Answer 3

I would suggest modifying the storage engine and schema to utilize foreign keys.
CREATE TABLE `article` (
`id` int(11) NOT NULL auto_increment,
`title` varchar(60) NOT NULL,
`time_stamp` int(11) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1000001 ;
CREATE TABLE `tag` (
`id` int(11) NOT NULL auto_increment,
`tag` varchar(30) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE `article_tag` (
`id` int(11) NOT NULL auto_increment,
`article_id` int(11) NOT NULL,
`tag_id` int(11) NOT NULL,
PRIMARY KEY (`id`),
FOREIGN KEY (`article_id`) REFERENCES article(id),
FOREIGN KEY (`tag_id`) REFERENCES tag(id)
) ENGINE=Innodb;
Which results in a query like so:
EXPLAIN
SELECT * FROM article
JOIN article_tag ON article.id = article_tag.id
JOIN tag ON article_tag.tag_id = tag.id
WHERE tag.tag="Acer";
+----+-------------+-------------+--------+----------------+---------+---------+-------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+--------+----------------+---------+---------+-------------------------+------+-------------+
| 1 | SIMPLE | article_tag | ALL | PRIMARY,tag_id | NULL | NULL | NULL | 1 | |
| 1 | SIMPLE | tag | eq_ref | PRIMARY | PRIMARY | 4 | temp.article_tag.tag_id | 1 | Using where |
| 1 | SIMPLE | article | eq_ref | PRIMARY | PRIMARY | 4 | temp.article_tag.id | 1 | |
+----+-------------+-------------+--------+----------------+---------+---------+-------------------------+------+-------------+
3 rows in set (0.00 sec)

Answer 4

Edit: Add this index
UNIQUE KEY tag (article_id,tag)