Question

I’ve downloaded the Tatoeba project databases and I’m trying to query them but queries with a subquery are taking way too long.

-- 800.000 rows approx.
CREATE TABLE `sentences` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `language` char(3) DEFAULT NULL,
  `text` mediumtext,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=912551 DEFAULT CHARSET=utf8


-- 1.5 million rows approx.
CREATE TABLE `links` (
  `sentenceId` int(10) unsigned NOT NULL,
  `translatedId` int(10) unsigned NOT NULL,
  PRIMARY KEY (`sentenceId`,`translatedId`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8

Basically the links table joins two sentences in the sentences table together (the original sentence and one translation). A sentence can have zero or more translations. So I have an id of a sentence I want to work with and want to grab ALL the translations available.

This query gets me what I want but takes almost 18 seconds to complete.

SELECT * FROM `sentences` WHERE `id` IN (SELECT `translatedId` FROM `links` WHERE `sentenceId` = 157967);

Running both queries by themselves just takes an instant.

What am I doing wrong?

Answer 1

SELECT `sentences`.* FROM 
  `sentences` JOIN 
  `links` ON `id` = `translatedId` 
WHERE `sentenceId` = 157967;

Some versions of MySQL are known not to use indexes in sub-queries.