Question

I am implementing ranking solution for one of my tables to optimize read queries to get rid of expensive queries which use COUNT(*), LIMIT and OFFSET clause. My problem is that I don’t know why position calculation are incorrect. Please look at my example to reproduce problem.

CREATE TABLE `acl`
(
    `id` MEDIUMINT(8) UNSIGNED NOT NULL AUTO_INCREMENT,
    `name` VARCHAR(32) NOT NULL,
    `limiter` INTEGER(11) SIGNED NULL,
    PRIMARY KEY (`id`)
)
ENGINE=INNODB;

CREATE TABLE `quote`
(
    `id` MEDIUMINT(8) UNSIGNED NOT NULL AUTO_INCREMENT,
    `created_at` INTEGER(11) UNSIGNED NOT NULL,
    `reputation` INTEGER(11) SIGNED NOT NULL,
    PRIMARY KEY (`id`)
)
ENGINE=INNODB;

INSERT INTO `acl` (`name`, `limiter`) VALUES ('Users', 0), ('Staff', null);
INSERT INTO `quote` (`created_at`, `reputation`) 
  VALUES (UNIX_TIMESTAMP(), 0), (UNIX_TIMESTAMP()+1, 0);

SET @acl_id := 0, @position := 0;
SELECT acl.id AS acl_id, quote.id AS quote_id, 
  GREATEST(@position := IF(@acl_id = acl.id, @position + 1, 1), 
    LEAST(0, @acl_id := acl.id)) AS position 
FROM acl JOIN quote 
  ON (acl.limiter IS NULL OR quote.reputation >= acl.limiter) 
ORDER BY acl.id ASC, quote.created_at DESC;

I would like that select query to fetch all acl rows and join them with quote rows at the same time, set their position, but all I get is position=1 for every row. Someone suggested me to move variable assignments to JOIN or ORDER clause but the problem remains. My question is… how to assign position in single query?

Answer 1

A slightly modified query from the article in my blog:

• Analytic functions: SUM, AVG, ROW_NUMBER

 

SELECT  q.*,
        @r := @r + 1
FROM    (
        SELECT  @_acl_id := -1,
                @r := 0
        ) vars
STRAIGHT_JOIN
        (
        SELECT  acl.id AS acl_id, quote.id AS quote_id
        FROM    acl
        JOIN    quote
        ON      (acl.limiter IS NULL OR quote.reputation >= acl.limiter)
        ORDER BY
                acl.id ASC, quote.created_at DESC
        ) q
WHERE   CASE WHEN @_acl_id <> acl_id THEN @r := 0 ELSE 0 END IS NOT NULL
        AND (@_acl_id := acl_id) IS NOT NULL