mysql 什么会更快

Question

Currently for my website, my articles have tags on them.

When I loop through the list of articles, for each one I do a mysql query to gather a list of category tags for them like so:

SELECT c.`category_name`, c.`category_id` 
FROM `articles_categorys` c 
    INNER JOIN `article_category_reference` r ON c.category_id = r.category_id 
WHERE r.article_id = 8136 
ORDER BY r.`category_id` = *IDHERE* DESC, r.`category_id` ASC 
LIMIT 4

Now, imagine a page where I have say 30 articles shown at once, the above query would be done 30 times. I imagine that's terrible.

What I decided to do was make an array of all the article_ids on a page and then do this instead:

SELECT c.`category_name`, c.`category_id`, r.article_id 
FROM `articles_categorys` c 
INNER JOIN `article_category_reference` r ON c.category_id = r.category_id 
WHERE r.article_id = 8136 OR r.article_id = 8130 OR r.article_id = 8127 
  OR r.article_id = 8125 OR r.article_id = 8123 OR r.article_id = 8120 
  OR r.article_id = 8119 OR r.article_id = 8117 OR r.article_id = 8116 
  OR r.article_id = 8112 OR r.article_id = 8107 OR r.article_id = 8106 
  OR r.article_id = 8037 OR r.article_id = 8104 OR r.article_id = 8103

I then use PHP just to loop through the array found from the above, match the article_id to the current article to them and if it matches echo the category_name. The only problem is now I can't limit it to 4 per article_id.

Is the newer approach better, worse, or do both suck horribly?

Example data to use:

CREATE TABLE `articles_categorys` (
  `category_id` int(11) NOT NULL,
  `category_name` varchar(32) COLLATE utf8_bin NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

CREATE TABLE `article_category_reference` (
  `ref_id` int(11) NOT NULL,
  `article_id` int(11) NOT NULL,
  `category_id` int(11) NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

INSERT INTO `articles_categorys` (`category_id`, `category_name`) VALUES
(22, 'Site Info'),
(1, 'Editorial'),
(2, 'Review'),
(3, 'Interview'),
(4, 'Game Sale'),
(5, 'Steam'),
(6, 'Indie Game'),
(7, 'Crowdfunding'),
(8, 'Game Bundle'),
(9, 'Free Game'),
(10, 'MMO');

INSERT INTO `article_category_reference` (`ref_id`, `article_id`, `category_id`) VALUES
(15, 6231, 22),
(14, 6231, 1),
(16, 6231, 2),
(17, 6231, 3),
(18, 6231, 4),
(19, 6231, 9),
(20, 6231, 10);

Answer 1

With SQL, it's always good to benchmark and do an EXPLAIN on your queries to see which will be faster. But as a rule of thumb, IN will probably outperform multiple ORs. That still leaves you with the problem of limiting the results to four each. Contrary to what's been said in the comments, it's not a good idea to fetch all the data and then choose only 4 items using PHP. You could potentially be retrieving thousands of rows only to display a hundred.

SET @rank=null, @val=null;
SELECT * FROM (

    SELECT c.`category_name`, c.`category_id`, r.article_id,
    @rank := IF(@val =  r.article_id, @rank+1,1) as rank,
    @val := r.article_id  
    FROM `articles_categorys` c 
    INNER JOIN `article_category_reference` r ON c.category_id = r.category_id 
    WHERE r.article_id IN (8136, /* fill this up accordingly */)

    ORDER BY r.`category_id` ASC
) AS a WHERE rank < 5;

For more details, see this excellent Q&A: Using LIMIT within GROUP BY to get N results per group?

In short, what this query does is to asign a rank to each record. At the start the rank is null, for the first row it will be set to null. While the r.category_id remains the same it will keep on increasing but would drop back to one when the category_id changes.

Answer 2

sometimes not enough to just make good query, you must use SQL options. To make your queries faster you should use indices (such as hash or btree) or unique values https://www.tutorialspoint.com/mysql/mysql-indexes.htm .