mysql - 为每个父优化选择最新的子

Question

我有 2 个表，一个关键字表和一个关键字位置表

关键词结构

CREATE TABLE `keywords`
(
    `id`      bigint(20) UNSIGNED                     NOT NULL AUTO_INCREMENT,
    `keyword` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL,
    PRIMARY KEY (`id`)
) ENGINE = InnoDB
  DEFAULT CHARSET = utf8mb4
  COLLATE = utf8mb4_unicode_ci;

关键字位置结构

CREATE TABLE `keyword_positions`
(
    `id`         bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,
    `keyword_id` bigint(20) UNSIGNED NOT NULL,
    `position`   int(11) DEFAULT NULL,
    PRIMARY KEY (`id`)
) ENGINE = InnoDB
  DEFAULT CHARSET = utf8mb4
  COLLATE = utf8mb4_unicode_ci;

这就是我执行选择的方式

SELECT `keywords`.*,
       kp.position AS kp_position
FROM `keywords`
         LEFT JOIN `keyword_positions` AS `kp`
                   ON `kp`.`keyword_id` = `keywords`.`id`
                       AND `kp`.`id` =
                           (SELECT MAX(ikp.id)
                            FROM keyword_positions AS ikp
                            WHERE keywords.id = ikp.keyword_id)

这个查询可以进一步优化吗？它运行大约 800 毫秒

Answer 1

好的，当我不使用时，我得到了更好的性能MAX，这是我的新查询

SELECT `keywords`.*,
       kp.position AS kp_position
FROM `keywords`
         left join `keyword_positions` as `kp`
                   on `kp`.`keyword_id` = `keywords`.`id`
                       and `kp`.`id` = (SELECT ikp.id
                                        FROM keyword_positions AS ikp
                                        WHERE keywords.id = ikp.keyword_id
                                        ORDER BY ikp.id DESC
                                        LIMIT 1)

运行 30-60 毫秒

Answer 2

ikp如果有，最里面的子查询可能会运行得更快

INDEX(keyword_id, id)

这样，优化器可以MAX()更有效地完成工作。