为每个父优化选择最新的子

如何解决为每个父优化选择最新的子

我有 2 个表，一个关键字表和一个关键字位置表

关键字结构

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;

keyword_positions 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;

这就是我如何执行我的选择

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

解决方法

好的，当我不使用 MAX 时，性能要好得多，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

,

如果有 xx <- c(3L,2L,1L,4L,3L,1L) #Initialise output vector yy <- integer(length(xx)) #Assign the 1st group yy[1:xx[1]] <- 1 #Set the current position i <- xx[1] + 1 #Initialise the group number group <- 2 #While all the groups have been assigned while(any(yy == 0)) { #Assign the next group number yy[i:(i+xx[i] - 1)] <- group #Increment the group number group <- group + 1 #Increment the current position. i <- i+xx[i] } yy #[1] 1 1 1 2 2 2 2 3 4 4 5 5 6 6 6 6 ，最里面的子查询可能会运行得更快

ikp

这样，优化器可以更高效地执行 INDEX(keyword_id,id) 。