Should we use the "LIMIT clause" in following example?

Question

There is a structure:
CREATE TABLE IF NOT EXISTS `categories` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `parent_id` int(11) unsigned NOT NULL DEFAULT '0',
  `title` varchar(255) NOT NULL,
  PRIMARY KEY (`id`),
) ENGINE=MyISAM  DEFAULT CHARSET=utf8;

Query_1:
SELECT * FROM `categories` WHERE `id` = 1234 
Query_2:
SELECT * FROM `categories` WHERE `id` = 1234 LIMIT 1

I need to get just one row. Since we apply WHERE id=1234 (finding by PRIMARY KEY) obviously that row with id=1234 is only one in whole table.
After MySQL has found the row, whether engine to continue the search when using Query_1?
Thanks in advance.

Answer 1

Look at this SQLFiddle: http://sqlfiddle.com/#!2/a8713/4 and especially View Execution Plan.

You see, that MySQL recognizes the predicate on a PRIMARY column and therefore it does not matter if you add LIMIT 1 or not.

PS: A little more explanation: Look at the column rows of the Execution Plan. The number there is the amount of columns, the query engine thinks, it has to examine. Since the columns content is unique (as it's a primary key), this is 1. Compare it to this: http://sqlfiddle.com/#!2/9868b/2 same schema but without primary key. Here rows says 8. (The execution plan is explained in the German MySQL reference, http://dev.mysql.com/doc/refman/5.1/en/explain.html the English one is for some reason not so detailed.)