Question

I have 2 table t1 -> t2 (common one to many relationship) with 140.000 records on table t2 reffering 50.000 records on t1, foreing key some times is null (no parent).

CREATE TABLE `t1` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(50) NOT NULL,
PRIMARY KEY (`id`),
KEY `name_idx` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=125666 DEFAULT CHARSET=utf8

CREATE TABLE `t2` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(50) NOT NULL,
`t1_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `t1_id_idx` (`t1_id`)
CONSTRAINT `t1_fk` FOREIGN KEY (`t1_id`) REFERENCES `t1` (`id`),
) ENGINE=InnoDB AUTO_INCREMENT=125666 DEFAULT CHARSET=utf8

This query running on 15 seconds:

SELECT * FROM t2
LEFT JOIN t1 ON t2.t1_id = t1.id
ORDER BY t1.name ASC
LIMIT 10; 

This query running on 0.5 seconds:

SELECT * FROM t2
LEFT JOIN t1 ON t2.t1_id = t1.id
WHERE t1.name <> 'any not found value'
ORDER BY t1.name ASC
LIMIT 10;

Can any body explain to me why this occurs?

Obs: Edited.

News:

This query running on 0.06 seconds: (WHAT’S CHANGE? inner join!!)

SELECT * FROM t2
INNER JOIN t1 ON t2.t1_id = t1.id
ORDER BY t1.name ASC
LIMIT 10; 

but above query does not is a solution for my, in my case t2.t1_id can be null some times.

Any Idea??

News:

Running explain with left and inner join:

Mysql show: Using temporary; Using filesort; Rows: 140.000

With Inner Join:

Mysql show: Using Where; Rows: 8

Answer 1

Solved!

The Problem is on order by, when using order by mysql create a temporaty file (Explain…Using Temporary), this temporary file is too big causing the lag.

Tips:

• Avoid Using Tempoaray
• When Using temporary don’t load much data.