MySQL索引用法

Question

I am performing a very simple select over a simple table, where the column that I am filtering over has an index.

Here is the schema:

 CREATE TABLE IF NOT EXISTS `tmp_inventory_items` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `transmission_id` int(11) unsigned NOT NULL,
  `inventory_item_id` int(11) unsigned DEFAULT NULL,
  `material_id` int(11) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  KEY `transmission_id` (`transmission_id`)
  KEY `inventory_item_id` (`inventory_item_id`),
  KEY `material_id` (`material_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=21 ;

Here is the SQL:

SELECT * FROM `tmp_inventory_items` WHERE `transmission_id` = 330

However, when explaining the query, I see that the index is NOT being used, why is that (the table has about 20 rows on my local machine)?

id  select_type table                   type    possible_keys   key     key_len ref     rows    Extra
1   SIMPLE      tmp_inventory_items...  ALL     transmission_id NULL    NULL    NULL    13      Using where

No key is being used even if I hint the mysql with USE INDEX(transmission_id)... this looks very strange to me (MySQL Version 5.5.28)

Answer 1

Because MySQL's algorithms tell it that preparing an index and using it would use more resources than simply performing the query without one.

When you feed query syntax to a DBMS, one of the things it does is attempts to determine the most efficient way to process the query (usually there are at least tens of ways).

If you want to, you can use FORCE INDEX(transmission_id) ( documented here ) which will inform MySQL that a table scan is assumed to be very expensive, but it's not recommended as to determine for 20 rows, it's just not valuable.