Question

I have a very simple table with two columns, but has 4.5M rows.

CREATE TABLE `content_link` (
  `category_id` mediumint(8) unsigned NOT NULL,
  `content_id` int(10) unsigned NOT NULL,
  PRIMARY KEY  (`content_id`,`category_id`),
  KEY `content_id` (`content_id`,`category_id`)
) ENGINE=MyISAM;

When I run a simple query like:

SELECT
    *   
FROM
    content_link
WHERE
    category_id = '11';

mysql spikes the CPU and takes 2-5 seconds before returning about 10 rows. The data is spread very evenly across the table and I’m accessing indexed fields (I’ve also analyzed/optimized the table and I never change the content of the table), so what reason is there for the query to take so long?

Edit: It seems navicat was lying to me and my primary key was not actually keyed in the right order as it was displaying the table to me.

Answer 1

category_id is not the first column in any index.

Recreate your secondary key as follows:

UNIQUE KEY `ix_category_id` (`category_id`, `content_id`)

Note the column order, it matters.