Question

I have the following table in a MySQL database:

CREATE TABLE `secondary_images` (
  `imgId` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `primaryId` int(10) unsigned DEFAULT NULL,
  `view` varchar(255) DEFAULT NULL,
  `imgURL` varchar(255) DEFAULT NULL,
  `imgDate` datetime DEFAULT NULL,
  PRIMARY KEY (`imgId`),
  KEY `primaryId` (`primaryId`),
  KEY `imgDate` (`imgDate`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 ;

The SQL will be the following:

SELECT imgURL, view FROM secondary_images 
WHERE primaryId={$imgId} ORDER BY imgDate DESC

As you can see I made both the primaryId and imgDate, Index Keys. My thinking behind that was because the WHERE clause queries results using the primaryId, and the ORDER clause uses imgDate.

My question is, would it be better to use Multiple Indexes as I am right now? Or should I a Multiple Column Index (something I don’t understand all too well at the moment)?

This is what I get from EXPLAIN:

id = 1   
select_type = simple      
table = secondary_images         
type = ref
possible_keys = primaryId
key = primaryId
key_len = 5
ref = const
rows = 1
extra = Using where; Using filesort

NOTE: This is not using a Multiple Column Index, it is the result from using the above table description.

Answer 1

You should use a multi-column index on (primaryId, imgDate) so that MySQL is able to use it for selecting the rows and sorting.

If all the columns used for sorting are not in the index used for selection, MySQL uses the “filesort” strategy, which consists of sorting all rows (in memory if there is not too much rows; on disk else).

If all columns used for sorting are in the index, MySQL uses the index to get the rows order (with some restrictions).

MySQL uses a tree structure for the indexes. This allows to access keys in order directly without sorting.

A multi-column index is basically an index of the concatenation of the columns. This allows MySQL to find the first row matching primaryId={$imgId}, and then access all the other rows directly in the right order.

With a single-row index on primaryId, MySQL can find all the rows matching primaryId={$imgId}, but it will find the rows in no particular order; so it will have to sort them after that.

See EXPLAIN and ORDER BY Optimization.