Using tuple comparison in mysql is it efficient?

问题

I have a table of books :

CREATE TABLE `books` (
    `id` INT(11) NOT NULL AUTO_INCREMENT,
    `nameOfBook` VARCHAR(32),
    `releaseDate` DATETIME NULL DEFAULT NULL,
    PRIMARY KEY (`id`),
    INDEX `Index 2` (`releaseDate`, `id`)
)
COLLATE='latin1_swedish_ci'
ENGINE=InnoDB

AUTO_INCREMENT=33029692;

I compared two SQL requests to do a pagiation with sort on releaseDate. Both of theses request return the same result.

(simple one)

select SQL_NO_CACHE  id,name, releaseDate  
from books  
where releaseDate <= '2016-11-07'  
AND (releaseDate<'2016-11-07' OR id <    3338191)  
ORDER  by releaseDate DESC, id DESC limit 50;

and

(tuple comparison or row comparaison)

select SQL_NO_CACHE  id,name, releaseDate 
from books 
where (releaseDate ,id) < ('2016-11-07',3338191) 
ORDER  by releaseDate DESC, id DESC limit 50;

When I do the explain of the request i got this

simple one :

"id";"select_type";"table";"type";"possible_keys";"key";"key_len";"ref";"rows";"Extra"
"1";"SIMPLE";"books";"range";"PRIMARY,Index 2";"Index 2";"9";"";"1015876";"Using where; Using index"

We can see it is parsing "1015876" of rows

The explain for the tuple comparison :

"id";"select_type";"table";"type";"possible_keys";"key";"key_len";"ref";"rows";"Extra"
"1";"SIMPLE";"books";"index";"";"Index 2";"13";"";"50";"Using where; Using index"

We can see it is parsing "50" of rows.

But if I checked the exectution time the simple one :

* Affected rows: 0  Lignes trouvées: 50  Avertissements: 0  Durée pour 1 query: 0,031 sec. */

and the tuple one :

/* Affected rows: 0  Lignes trouvées: 50  Avertissements: 0  Durée pour 1 query: 3,682 sec. */

I don't understant why according to the explain the tuple comparison is better but the execution time is badly worse?

回答1:

I've been irritated by this for years. WHERE (a,b) > (1,2) has never been optimized, in spite of it being easily transformed into the other formulation. Even the other format was poorly optimized until a few years ago.

Using EXPLAIN FORMAT=JSON SELECT ... might give you some better clues.

Meanwhile, EXPLAIN ignored the LIMIT and suggested 1015876. On many cases, EXPLAIN provides a "decent" Row estimate, but not either of these.

Feel free to file a bug report: http://bugs.mysql.com (and post the link here).

Another formulation was recently optimized, in spite of OR being historically un-optimizable.

where  releaseDate <  '2016-11-07'  
   OR (releaseDate  = '2016-11-07' AND id < 3338191)

For measuring query optimizations, I like to do:

FLUSH STATUS;
SELECT ...
SHOW SESSION STATUS LIKE 'Handler%';

Small values, such as '50' for your case, indicate good optimization; large value (1M) indicate a scan. The Handler numbers are exact; unlike the estimates in EXPLAIN.

Update 5.7.3 has improved handling of tuples, aka "row constructors"

来源：https://stackoverflow.com/questions/40489417/using-tuple-comparison-in-mysql-is-it-efficient

标签

mysql

tuples

sqlperformance