SQL 单行视图（带 where 子句）
SQL Views for single row (with where clause)
我正在开发一个管理课程的系统，有多个 类，有多个课程和多个消费者。随着系统的增长，需要更多的数据，因此由于一些性能问题，我决定使用 SQL 视图。我们正在使用 MySQL.
所以我已经替换了对数据库的旧调用（例如对于单节课）
select * from `courses_classes_lessons` where `courses_classes_lessons`.`deleted_at` is null limit 1;
select count(consumer_id) as consumers_count from `courses_classes_lessons_consumers` where `lesson_id` = '448' limit 1;
select `max_consumers` from `courses_classes` where `id` = '65' limit 1;
select `id` from `courses_classes_lessons` left join `courses_classes_lessons_consumers` on `courses_classes_lessons_consumers`.`lesson_id` = `courses_classes_lessons`.`id` where `id` = '448' group by `courses_classes_lessons`.`id` having count(courses_classes_lessons_consumers.consumer_id) < '4' limit 1;
select courses_classes.max_consumers - LEAST(count(courses_classes_lessons_consumers.consumer_id), courses_classes.max_consumers) as available_spaces from `courses_classes_lessons` left join `courses_classes_lessons_consumers` on `courses_classes_lessons_consumers`.`lesson_id` = `courses_classes_lessons`.`id` left join `courses_classes` on `courses_classes_lessons`.`class_id` = `courses_classes`.`id` where `courses_classes_lessons`.`id` = '448' group by `courses_classes`.`id` limit 1;
以上花费了大约 4-5 毫秒
与SQL查看如下：
CREATE OR REPLACE VIEW `courses_classes_lessons_view` AS
SELECT
courses_classes_lessons.id AS lesson_id,
(SELECT
max_consumers
FROM
courses_classes
WHERE
id = courses_classes_lessons.class_id
LIMIT 1) AS class_max_consumers,
(SELECT
count(consumer_id)
FROM
courses_classes_lessons_consumers
WHERE
lesson_id = courses_classes_lessons.id) AS consumers_count,
(SELECT
CASE WHEN consumers_count >= class_max_consumers THEN
TRUE
ELSE
FALSE
END AS is_full) AS is_full,
(CASE WHEN courses_classes_lessons.completed_at > NOW() THEN
'completed'
WHEN courses_classes_lessons.cancelled_at > NOW() THEN
'cancelled'
WHEN courses_classes_lessons.starts_at > NOW() THEN
'upcoming'
ELSE
'incomplete'
END) AS status,
(SELECT
class_max_consumers - LEAST(consumers_count, class_max_consumers)) AS available_spaces
FROM
courses_classes_lessons
我遇到的问题是，无论我是加载整个视图还是从其中加载一行都没有关系 - 加载总是需要大约 6-9 秒！但是，当我使用 WHERE 子句尝试相同的查询时，它需要大约 500 微秒。我是 SQL View 的新手并感到困惑 - 为什么没有 indexes/primary 键可用于快速加载单行？我做错了什么吗？
解释结果
INSERT INTO `courses_classes` (`id`, `select_type`, `table`, `partitions`, `type`, `possible_keys`, `key`, `key_len`, `ref`, `rows`, `filtered`, `Extra`) VALUES
(1, 'PRIMARY', 'courses_classes_lessons', NULL, 'ALL', NULL, NULL, NULL, NULL, 478832, 100.00, NULL),
(3, 'DEPENDENT SUBQUERY', 'courses_classes_lessons_consumers', NULL, 'ref', 'PRIMARY,courses_classes_lessons_consumers_lesson_id_index', 'courses_classes_lessons_consumers_lesson_id_index', '4', 'api.courses_classes_lessons.id', 3, 100.00, 'Using index'),
(2, 'DEPENDENT SUBQUERY', 'courses_classes', NULL, 'eq_ref', 'PRIMARY,courses_classes_id_parent_id_index', 'PRIMARY', '4', 'api.courses_classes_lessons.class_id', 1, 100.00, NULL);
TABLE 结构
课程
CREATE TABLE `courses_classes_lessons` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`franchisee_id` int(10) unsigned NOT NULL,
`class_id` int(10) unsigned NOT NULL,
`instructor_id` int(10) unsigned NOT NULL,
`instructor_rate` int(10) unsigned NOT NULL DEFAULT '0',
`instructor_total` int(10) unsigned NOT NULL DEFAULT '0',
`instructor_paid` tinyint(1) NOT NULL DEFAULT '0',
`starts_at` timestamp NULL DEFAULT NULL,
`ends_at` timestamp NULL DEFAULT NULL,
`completed_at` timestamp NULL DEFAULT NULL,
`cancelled_at` timestamp NULL DEFAULT NULL,
`cancelled_reason` varchar(255) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
`cancelled_reason_extra` varchar(255) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
`created_at` timestamp NULL DEFAULT NULL,
`updated_at` timestamp NULL DEFAULT NULL,
`deleted_at` timestamp NULL DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `courses_classes_lessons_franchisee_id_foreign` (`franchisee_id`),
KEY `courses_classes_lessons_class_id_foreign` (`class_id`),
KEY `courses_classes_lessons_instructor_id_foreign` (`instructor_id`),
KEY `courses_classes_lessons_starts_at_ends_at_index` (`starts_at`,`ends_at`),
KEY `courses_classes_lessons_completed_at_index` (`completed_at`),
KEY `courses_classes_lessons_cancelled_at_index` (`cancelled_at`),
KEY `courses_classes_lessons_class_id_deleted_at_index` (`class_id`,`deleted_at`),
KEY `courses_classes_lessons_deleted_at_index` (`deleted_at`),
KEY `class_ownership_index` (`class_id`,`starts_at`,`cancelled_at`,`deleted_at`),
CONSTRAINT `courses_classes_lessons_class_id_foreign` FOREIGN KEY (`class_id`) REFERENCES `courses_classes` (`id`) ON DELETE CASCADE,
CONSTRAINT `courses_classes_lessons_franchisee_id_foreign` FOREIGN KEY (`franchisee_id`) REFERENCES `franchisees` (`id`) ON DELETE CASCADE,
CONSTRAINT `courses_classes_lessons_instructor_id_foreign` FOREIGN KEY (`instructor_id`) REFERENCES `instructors` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=487853 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
教训消费者
CREATE TABLE `courses_classes_lessons_consumers` (
`lesson_id` int(10) unsigned NOT NULL,
`consumer_id` int(10) unsigned NOT NULL,
`present` tinyint(1) DEFAULT NULL,
`plan_id` int(10) unsigned DEFAULT NULL,
`created_at` timestamp NULL DEFAULT NULL,
`updated_at` timestamp NULL DEFAULT NULL,
PRIMARY KEY (`lesson_id`,`consumer_id`),
KEY `courses_classes_lessons_consumers_consumer_id_foreign` (`consumer_id`),
KEY `courses_classes_lessons_consumers_plan_id_foreign` (`plan_id`),
KEY `courses_classes_lessons_consumers_lesson_id_index` (`lesson_id`),
KEY `courses_classes_lessons_consumers_present_index` (`present`),
CONSTRAINT `courses_classes_lessons_consumers_consumer_id_foreign` FOREIGN KEY (`consumer_id`) REFERENCES `customers_consumers` (`id`) ON DELETE CASCADE,
CONSTRAINT `courses_classes_lessons_consumers_lesson_id_foreign` FOREIGN KEY (`lesson_id`) REFERENCES `courses_classes_lessons` (`id`) ON DELETE CASCADE,
CONSTRAINT `courses_classes_lessons_consumers_plan_id_foreign` FOREIGN KEY (`plan_id`) REFERENCES `customers_plans` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
从 类 开始，它只使用 max_consumers int(10) unsigned NOT NULL DEFAULT '0',
更新 1
我已将 SQL 视图更改为以下视图：
CREATE OR REPLACE VIEW `courses_classes_lessons_view` AS
SELECT
courses_classes_lessons.id AS lesson_id,
courses_classes.max_consumers AS class_max_consumers,
lessons_consumers.consumers_count AS consumers_count,
(
SELECT
CASE WHEN consumers_count >= class_max_consumers THEN
TRUE
ELSE
FALSE
END AS is_full) AS is_full,
(
CASE WHEN courses_classes_lessons.completed_at > NOW() THEN
'completed'
WHEN courses_classes_lessons.cancelled_at > NOW() THEN
'cancelled'
WHEN courses_classes_lessons.starts_at > NOW() THEN
'upcoming'
ELSE
'incomplete'
END) AS status,
(
SELECT
class_max_consumers - LEAST(consumers_count, class_max_consumers)) AS available_spaces
FROM
courses_classes_lessons
JOIN courses_classes ON courses_classes.id = courses_classes_lessons.class_id
JOIN (
SELECT
lesson_id,
count(*) AS consumers_count
FROM
courses_classes_lessons_consumers
GROUP BY
courses_classes_lessons_consumers.lesson_id) AS lessons_consumers ON lessons_consumers.lesson_id = courses_classes_lessons.id;
尽管 SELECT 查询本身似乎比前一个查询慢得多，但作为视图，它似乎执行得更好。它仍然没有我希望的那么快，但它是向前迈出的一步。
整体改进从6-7秒跳到800毫秒左右，这里的目标是在500微秒-1毫秒的范围内。有什么建议可以改进 SQL 查看更多吗？
更新 2
好的，我找到瓶颈了！再次 - 它有点类似于上一个（SELECT 查询对单行工作得很快，但是 SQL VIEW 每次都试图访问整个 table。
我的新课SQL查看：
CREATE OR REPLACE VIEW `courses_classes_lessons_view` AS
SELECT
courses_classes_lessons.id AS lesson_id,
courses_classes.max_consumers AS class_max_consumers,
IFNULL(lessons_consumers.consumers_count,0) AS consumers_count,
(
SELECT
CASE WHEN consumers_count >= class_max_consumers THEN
TRUE
ELSE
FALSE
END AS is_full) AS is_full,
(
CASE WHEN courses_classes_lessons.completed_at > NOW() THEN
'completed'
WHEN courses_classes_lessons.cancelled_at > NOW() THEN
'cancelled'
WHEN courses_classes_lessons.starts_at > NOW() THEN
'upcoming'
ELSE
'incomplete'
END) AS status,
(
SELECT
IFNULL(class_max_consumers, 0) - LEAST(IFNULL(consumers_count,0), class_max_consumers)) AS available_spaces
FROM
courses_classes_lessons
JOIN courses_classes ON courses_classes.id = courses_classes_lessons.class_id
LEFT JOIN courses_classes_lessons_consumers_view AS lessons_consumers ON lessons_consumers.lesson_id = courses_classes_lessons.id;
另一个 SQL 视图 - 这次是针对消费者的：
CREATE OR REPLACE VIEW `courses_classes_lessons_consumers_view` AS
SELECT
lesson_id,
IFNULL(count(
consumer_id),0) AS consumers_count
FROM
courses_classes_lessons_consumers
GROUP BY
courses_classes_lessons_consumers.lesson_id;
看来这个人就是麻烦制造者！消费者table在上面，这里是对上面SELECT查询的解释：
INSERT INTO `courses_classes_lessons_consumers` (`id`, `select_type`, `table`, `partitions`, `type`, `possible_keys`, `key`, `key_len`, `ref`, `rows`, `filtered`, `Extra`)
VALUES(1, 'SIMPLE', 'courses_classes_lessons_consumers', NULL, 'index', 'PRIMARY,courses_classes_lessons_consumers_consumer_id_foreign,courses_classes_lessons_consumers_plan_id_foreign,courses_classes_lessons_consumers_lesson_id_index,courses_classes_lessons_consumers_present_index', 'courses_classes_lessons_consumers_lesson_id_index', '4', NULL, 1330649, 100.00, 'Using index');
知道如何展开这个计数吗？
考虑编写存储过程；它可能能够让 448 到位以更好地优化。
- 如果您知道只有一行（例如在执行
COUNT(*) 时），请跳过 LIMIT 1。
- 除非
consumer_id 可能是 NULL，否则请使用 COUNT(*) 而不是 COUNT(consumer_id)。
- 没有
ORDER BY 的 LIMIT 会使您获得随机行。
- 如果
courses_classes_lessons_consumers是一个多对多的映射table，我看到SHOW CREATE TABLE. 后可能会有一些索引建议
- 5 个
SELECTs 中哪个最慢？
经过多次尝试，过程方式似乎是最好的方法，我不会在 SQL 视图上花费更多时间
这是我写的程序：
CREATE PROCEDURE `LessonData`(
IN lessonId INT(10)
)
BEGIN
SELECT
courses_classes_lessons.id AS lesson_id,
courses_classes.max_consumers AS class_max_consumers,
IFNULL((SELECT
count(consumer_id) as consumers_count
FROM
courses_classes_lessons_consumers
WHERE
lesson_id = courses_classes_lessons.id
GROUP BY
courses_classes_lessons_consumers.lesson_id), 0) AS consumers_count,
(
SELECT
CASE WHEN consumers_count >= class_max_consumers THEN
TRUE
ELSE
FALSE
END) AS is_full,
(
CASE WHEN courses_classes_lessons.completed_at > NOW() THEN
'completed'
WHEN courses_classes_lessons.cancelled_at > NOW() THEN
'cancelled'
WHEN courses_classes_lessons.starts_at > NOW() THEN
'upcoming'
ELSE
'incomplete'
END) AS status,
(
SELECT
class_max_consumers - LEAST(consumers_count, class_max_consumers)) AS available_spaces
FROM
courses_classes_lessons
JOIN courses_classes ON courses_classes.id = courses_classes_lessons.class_id
WHERE courses_classes_lessons.id = lessonId;
END
执行时间约为500μs-1ms
谢谢大家的帮助！
我正在开发一个管理课程的系统，有多个 类，有多个课程和多个消费者。随着系统的增长，需要更多的数据，因此由于一些性能问题，我决定使用 SQL 视图。我们正在使用 MySQL.
所以我已经替换了对数据库的旧调用（例如对于单节课）
select * from `courses_classes_lessons` where `courses_classes_lessons`.`deleted_at` is null limit 1;
select count(consumer_id) as consumers_count from `courses_classes_lessons_consumers` where `lesson_id` = '448' limit 1;
select `max_consumers` from `courses_classes` where `id` = '65' limit 1;
select `id` from `courses_classes_lessons` left join `courses_classes_lessons_consumers` on `courses_classes_lessons_consumers`.`lesson_id` = `courses_classes_lessons`.`id` where `id` = '448' group by `courses_classes_lessons`.`id` having count(courses_classes_lessons_consumers.consumer_id) < '4' limit 1;
select courses_classes.max_consumers - LEAST(count(courses_classes_lessons_consumers.consumer_id), courses_classes.max_consumers) as available_spaces from `courses_classes_lessons` left join `courses_classes_lessons_consumers` on `courses_classes_lessons_consumers`.`lesson_id` = `courses_classes_lessons`.`id` left join `courses_classes` on `courses_classes_lessons`.`class_id` = `courses_classes`.`id` where `courses_classes_lessons`.`id` = '448' group by `courses_classes`.`id` limit 1;
以上花费了大约 4-5 毫秒
与SQL查看如下：
CREATE OR REPLACE VIEW `courses_classes_lessons_view` AS
SELECT
courses_classes_lessons.id AS lesson_id,
(SELECT
max_consumers
FROM
courses_classes
WHERE
id = courses_classes_lessons.class_id
LIMIT 1) AS class_max_consumers,
(SELECT
count(consumer_id)
FROM
courses_classes_lessons_consumers
WHERE
lesson_id = courses_classes_lessons.id) AS consumers_count,
(SELECT
CASE WHEN consumers_count >= class_max_consumers THEN
TRUE
ELSE
FALSE
END AS is_full) AS is_full,
(CASE WHEN courses_classes_lessons.completed_at > NOW() THEN
'completed'
WHEN courses_classes_lessons.cancelled_at > NOW() THEN
'cancelled'
WHEN courses_classes_lessons.starts_at > NOW() THEN
'upcoming'
ELSE
'incomplete'
END) AS status,
(SELECT
class_max_consumers - LEAST(consumers_count, class_max_consumers)) AS available_spaces
FROM
courses_classes_lessons
我遇到的问题是，无论我是加载整个视图还是从其中加载一行都没有关系 - 加载总是需要大约 6-9 秒！但是，当我使用 WHERE 子句尝试相同的查询时，它需要大约 500 微秒。我是 SQL View 的新手并感到困惑 - 为什么没有 indexes/primary 键可用于快速加载单行？我做错了什么吗？
解释结果
INSERT INTO `courses_classes` (`id`, `select_type`, `table`, `partitions`, `type`, `possible_keys`, `key`, `key_len`, `ref`, `rows`, `filtered`, `Extra`) VALUES
(1, 'PRIMARY', 'courses_classes_lessons', NULL, 'ALL', NULL, NULL, NULL, NULL, 478832, 100.00, NULL),
(3, 'DEPENDENT SUBQUERY', 'courses_classes_lessons_consumers', NULL, 'ref', 'PRIMARY,courses_classes_lessons_consumers_lesson_id_index', 'courses_classes_lessons_consumers_lesson_id_index', '4', 'api.courses_classes_lessons.id', 3, 100.00, 'Using index'),
(2, 'DEPENDENT SUBQUERY', 'courses_classes', NULL, 'eq_ref', 'PRIMARY,courses_classes_id_parent_id_index', 'PRIMARY', '4', 'api.courses_classes_lessons.class_id', 1, 100.00, NULL);
TABLE 结构
课程
CREATE TABLE `courses_classes_lessons` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`franchisee_id` int(10) unsigned NOT NULL,
`class_id` int(10) unsigned NOT NULL,
`instructor_id` int(10) unsigned NOT NULL,
`instructor_rate` int(10) unsigned NOT NULL DEFAULT '0',
`instructor_total` int(10) unsigned NOT NULL DEFAULT '0',
`instructor_paid` tinyint(1) NOT NULL DEFAULT '0',
`starts_at` timestamp NULL DEFAULT NULL,
`ends_at` timestamp NULL DEFAULT NULL,
`completed_at` timestamp NULL DEFAULT NULL,
`cancelled_at` timestamp NULL DEFAULT NULL,
`cancelled_reason` varchar(255) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
`cancelled_reason_extra` varchar(255) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
`created_at` timestamp NULL DEFAULT NULL,
`updated_at` timestamp NULL DEFAULT NULL,
`deleted_at` timestamp NULL DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `courses_classes_lessons_franchisee_id_foreign` (`franchisee_id`),
KEY `courses_classes_lessons_class_id_foreign` (`class_id`),
KEY `courses_classes_lessons_instructor_id_foreign` (`instructor_id`),
KEY `courses_classes_lessons_starts_at_ends_at_index` (`starts_at`,`ends_at`),
KEY `courses_classes_lessons_completed_at_index` (`completed_at`),
KEY `courses_classes_lessons_cancelled_at_index` (`cancelled_at`),
KEY `courses_classes_lessons_class_id_deleted_at_index` (`class_id`,`deleted_at`),
KEY `courses_classes_lessons_deleted_at_index` (`deleted_at`),
KEY `class_ownership_index` (`class_id`,`starts_at`,`cancelled_at`,`deleted_at`),
CONSTRAINT `courses_classes_lessons_class_id_foreign` FOREIGN KEY (`class_id`) REFERENCES `courses_classes` (`id`) ON DELETE CASCADE,
CONSTRAINT `courses_classes_lessons_franchisee_id_foreign` FOREIGN KEY (`franchisee_id`) REFERENCES `franchisees` (`id`) ON DELETE CASCADE,
CONSTRAINT `courses_classes_lessons_instructor_id_foreign` FOREIGN KEY (`instructor_id`) REFERENCES `instructors` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=487853 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
教训消费者
CREATE TABLE `courses_classes_lessons_consumers` (
`lesson_id` int(10) unsigned NOT NULL,
`consumer_id` int(10) unsigned NOT NULL,
`present` tinyint(1) DEFAULT NULL,
`plan_id` int(10) unsigned DEFAULT NULL,
`created_at` timestamp NULL DEFAULT NULL,
`updated_at` timestamp NULL DEFAULT NULL,
PRIMARY KEY (`lesson_id`,`consumer_id`),
KEY `courses_classes_lessons_consumers_consumer_id_foreign` (`consumer_id`),
KEY `courses_classes_lessons_consumers_plan_id_foreign` (`plan_id`),
KEY `courses_classes_lessons_consumers_lesson_id_index` (`lesson_id`),
KEY `courses_classes_lessons_consumers_present_index` (`present`),
CONSTRAINT `courses_classes_lessons_consumers_consumer_id_foreign` FOREIGN KEY (`consumer_id`) REFERENCES `customers_consumers` (`id`) ON DELETE CASCADE,
CONSTRAINT `courses_classes_lessons_consumers_lesson_id_foreign` FOREIGN KEY (`lesson_id`) REFERENCES `courses_classes_lessons` (`id`) ON DELETE CASCADE,
CONSTRAINT `courses_classes_lessons_consumers_plan_id_foreign` FOREIGN KEY (`plan_id`) REFERENCES `customers_plans` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
从 类 开始，它只使用 max_consumers int(10) unsigned NOT NULL DEFAULT '0',
更新 1
我已将 SQL 视图更改为以下视图：
CREATE OR REPLACE VIEW `courses_classes_lessons_view` AS
SELECT
courses_classes_lessons.id AS lesson_id,
courses_classes.max_consumers AS class_max_consumers,
lessons_consumers.consumers_count AS consumers_count,
(
SELECT
CASE WHEN consumers_count >= class_max_consumers THEN
TRUE
ELSE
FALSE
END AS is_full) AS is_full,
(
CASE WHEN courses_classes_lessons.completed_at > NOW() THEN
'completed'
WHEN courses_classes_lessons.cancelled_at > NOW() THEN
'cancelled'
WHEN courses_classes_lessons.starts_at > NOW() THEN
'upcoming'
ELSE
'incomplete'
END) AS status,
(
SELECT
class_max_consumers - LEAST(consumers_count, class_max_consumers)) AS available_spaces
FROM
courses_classes_lessons
JOIN courses_classes ON courses_classes.id = courses_classes_lessons.class_id
JOIN (
SELECT
lesson_id,
count(*) AS consumers_count
FROM
courses_classes_lessons_consumers
GROUP BY
courses_classes_lessons_consumers.lesson_id) AS lessons_consumers ON lessons_consumers.lesson_id = courses_classes_lessons.id;
尽管 SELECT 查询本身似乎比前一个查询慢得多，但作为视图，它似乎执行得更好。它仍然没有我希望的那么快，但它是向前迈出的一步。
整体改进从6-7秒跳到800毫秒左右，这里的目标是在500微秒-1毫秒的范围内。有什么建议可以改进 SQL 查看更多吗？
更新 2
好的，我找到瓶颈了！再次 - 它有点类似于上一个（SELECT 查询对单行工作得很快，但是 SQL VIEW 每次都试图访问整个 table。
我的新课SQL查看：
CREATE OR REPLACE VIEW `courses_classes_lessons_view` AS
SELECT
courses_classes_lessons.id AS lesson_id,
courses_classes.max_consumers AS class_max_consumers,
IFNULL(lessons_consumers.consumers_count,0) AS consumers_count,
(
SELECT
CASE WHEN consumers_count >= class_max_consumers THEN
TRUE
ELSE
FALSE
END AS is_full) AS is_full,
(
CASE WHEN courses_classes_lessons.completed_at > NOW() THEN
'completed'
WHEN courses_classes_lessons.cancelled_at > NOW() THEN
'cancelled'
WHEN courses_classes_lessons.starts_at > NOW() THEN
'upcoming'
ELSE
'incomplete'
END) AS status,
(
SELECT
IFNULL(class_max_consumers, 0) - LEAST(IFNULL(consumers_count,0), class_max_consumers)) AS available_spaces
FROM
courses_classes_lessons
JOIN courses_classes ON courses_classes.id = courses_classes_lessons.class_id
LEFT JOIN courses_classes_lessons_consumers_view AS lessons_consumers ON lessons_consumers.lesson_id = courses_classes_lessons.id;
另一个 SQL 视图 - 这次是针对消费者的：
CREATE OR REPLACE VIEW `courses_classes_lessons_consumers_view` AS
SELECT
lesson_id,
IFNULL(count(
consumer_id),0) AS consumers_count
FROM
courses_classes_lessons_consumers
GROUP BY
courses_classes_lessons_consumers.lesson_id;
看来这个人就是麻烦制造者！消费者table在上面，这里是对上面SELECT查询的解释：
INSERT INTO `courses_classes_lessons_consumers` (`id`, `select_type`, `table`, `partitions`, `type`, `possible_keys`, `key`, `key_len`, `ref`, `rows`, `filtered`, `Extra`)
VALUES(1, 'SIMPLE', 'courses_classes_lessons_consumers', NULL, 'index', 'PRIMARY,courses_classes_lessons_consumers_consumer_id_foreign,courses_classes_lessons_consumers_plan_id_foreign,courses_classes_lessons_consumers_lesson_id_index,courses_classes_lessons_consumers_present_index', 'courses_classes_lessons_consumers_lesson_id_index', '4', NULL, 1330649, 100.00, 'Using index');
知道如何展开这个计数吗？
考虑编写存储过程；它可能能够让 448 到位以更好地优化。
- 如果您知道只有一行（例如在执行
COUNT(*)时），请跳过LIMIT 1。 - 除非
consumer_id可能是NULL，否则请使用COUNT(*)而不是COUNT(consumer_id)。 - 没有
ORDER BY的LIMIT会使您获得随机行。 - 如果
courses_classes_lessons_consumers是一个多对多的映射table，我看到SHOW CREATE TABLE. 后可能会有一些索引建议
- 5 个
SELECTs中哪个最慢？
经过多次尝试，过程方式似乎是最好的方法，我不会在 SQL 视图上花费更多时间
这是我写的程序：
CREATE PROCEDURE `LessonData`(
IN lessonId INT(10)
)
BEGIN
SELECT
courses_classes_lessons.id AS lesson_id,
courses_classes.max_consumers AS class_max_consumers,
IFNULL((SELECT
count(consumer_id) as consumers_count
FROM
courses_classes_lessons_consumers
WHERE
lesson_id = courses_classes_lessons.id
GROUP BY
courses_classes_lessons_consumers.lesson_id), 0) AS consumers_count,
(
SELECT
CASE WHEN consumers_count >= class_max_consumers THEN
TRUE
ELSE
FALSE
END) AS is_full,
(
CASE WHEN courses_classes_lessons.completed_at > NOW() THEN
'completed'
WHEN courses_classes_lessons.cancelled_at > NOW() THEN
'cancelled'
WHEN courses_classes_lessons.starts_at > NOW() THEN
'upcoming'
ELSE
'incomplete'
END) AS status,
(
SELECT
class_max_consumers - LEAST(consumers_count, class_max_consumers)) AS available_spaces
FROM
courses_classes_lessons
JOIN courses_classes ON courses_classes.id = courses_classes_lessons.class_id
WHERE courses_classes_lessons.id = lessonId;
END
执行时间约为500μs-1ms
谢谢大家的帮助！