在我们的 MySQL 性能审查期间，我们看到我们的代码中有一些全表扫描。我们创建了一个跨越不同表的视图，这些表是主-从记录。例如，销售和销售线。
在这个主明细记录上，我们需要添加分页。所以我们必须能够获取前 10 个销售（以及它们相应的销售线）。还要获取前 10 个销售线，其中 1 个单价高于 10 美元。
所以我们创建了一个视图，在这个场景中是 vw_sales，它连接了销售和销售线，我们能够满足这些要求。
创建表可以在这里查看
CREATE TABLE `sale_test` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`amount` decimal(20,2) NOT NULL,
`created_on` datetime NOT NULL,
`tenant_id` bigint(20) NOT NULL,
`version` int(11) NOT NULL,
`timestamp` datetime NOT NULL,
`culture` varchar(10) NOT NULL,
PRIMARY KEY (`id`),
KEY `fk_sale_on_tenant` (`tenant_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
CREATE TABLE `sales_line_test` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`sale_id` bigint(20) NOT NULL,
`quantity` bigint(20) NOT NULL,
`unit_price` decimal(20,2) NOT NULL,
`offer_product_id` bigint(20) NOT NULL,
`product_id` bigint(20) DEFAULT NULL,
`tenant_id` bigint(20) NOT NULL,
`version` int(11) NOT NULL,
`timestamp` datetime NOT NULL,
PRIMARY KEY (`id`),
KEY `fk_sales_line_on_sale` (`sale_id`),
KEY `fk_sales_line_on_product` (`offer_product_id`),
KEY `fk_sales_line_on_tenant` (`tenant_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
视图创建为here
CREATE OR REPLACE
ALGORITHM = MERGE
DEFINER = `root`@`localhost`
SQL SECURITY DEFINER
VIEW `vw_sales` AS
SELECT
`sale_test`.`id` AS `id`,
`sale_test`.`amount` AS `amount`,
`sale_test`.`created_on` AS `createdon`,
`sale_test`.`tenant_id` AS `tenantid`,
`sales_line_test`.`id` AS `saleslineid`,
`sales_line_test`.`quantity` AS `quantity`,
`sales_line_test`.`unit_price` AS `unitprice`
FROM
`sale_test` FORCE INDEX (`fk_sale_on_tenant`)
INNER JOIN `sales_line_test` `sales_line_test` ON (`sales_line_test`.`sale_id` = `sale_test`.`id`);
我们添加了如下分页
SELECT
*,
CASE
WHEN @previous = id THEN @rank
WHEN @previous:=id THEN @rank:=@rank + 1
END AS rank
FROM
vw_sales,
(SELECT @previous:=0) previous,
(SELECT @rank:=- 1) rank
WHERE
id = 26
ORDER BY id ASC
解释命令的结果如下所示
+----+-------------+-----------------+------------+--------+-----------------------+-----------------------+---------+-------------------------------+------+----------+------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------------+------------+--------+-----------------------+-----------------------+---------+-------------------------------+------+----------+------------------+
| 1 | PRIMARY | <derived2> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | "Using filesort" |
| 1 | PRIMARY | <derived3> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 1 | PRIMARY | <derived4> | NULL | ref | <auto_key0> | <auto_key0> | 8 | const | 1 | 100.00 | "Using where" |
| 4 | DERIVED | sale_test | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 4 | DERIVED | sales_line_test | NULL | ref | fk_sales_line_on_sale | fk_sales_line_on_sale | 8 | order_management.sale_test.id | 1 | 100.00 | NULL |
| 3 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | "No tables used" |
| 2 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | "No tables used" |
+----+-------------+-----------------+------------+--------+-----------------------+-----------------------+---------+-------------------------------+------+----------+------------------+
但是我们看到，由于 MySql（物化视图）的限制，我们导致了一些全表扫描 - 表 sale_test。（参考。如何让 MySQL 使用 INDEX 进行视图查询？，MySQL 视图是否总是进行全表扫描？）
跨表创建覆盖索引似乎是不可能的，所以这个选项也是不可能的。
我们已经在视图中添加了 force 索引以提示正确的索引，但这并不能解决问题。
这是如何以下降的方式解决的？创建一个临时表不会造成很多开销吗？这看起来很简单，但是分页要求使它变得非常困难。我们正在使用 mysql 5.7 (AWS RDS)，因此在 mysql 8.0 中添加的窗口函数对我们来说不是一个选项。
仅供参考：如果没有视图，我们会看到正在使用索引
询问：
SELECT
`sale_test`.`id` AS `id`,
`sale_test`.`amount` AS `amount`,
`sale_test`.`created_on` AS `createdon`,
`sale_test`.`tenant_id` AS `tenantid`,
`sales_line_test`.`id` AS `saleslineid`,
`sales_line_test`.`quantity` AS `quantity`,
`sales_line_test`.`unit_price` AS `unitprice`,
CASE
WHEN @previous = `sale_test`.`id` THEN @rank
WHEN @previous:= `sale_test`.`id` THEN @rank:=@rank + 1
END AS rank
FROM
`sale_test` FORCE INDEX (FK_SALE_ON_TENANT)
INNER JOIN
`sales_line_test` `sales_line_test` ON (`sales_line_test`.`sale_id` = `sale_test`.`id`),
(SELECT @previous:=0) previous,
(SELECT @rank:=- 1) rank
WHERE
`sale_test`.`tenant_id` = 26
结果
+----+-------------+-----------------+------------+--------+-----------------------+-----------------------+---------+-------------------------------+------+----------+------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------------+------------+--------+-----------------------+-----------------------+---------+-------------------------------+------+----------+------------------+
| 1 | PRIMARY | <derived2> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 1 | PRIMARY | <derived3> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 1 | PRIMARY | sale_test | NULL | ref | fk_sale_on_tenant | fk_sale_on_tenant | 8 | const | 1 | 100.00 | NULL |
| 1 | PRIMARY | sales_line_test | NULL | ref | fk_sales_line_on_sale | fk_sales_line_on_sale | 8 | order_management.sale_test.id | 1 | 100.00 | NULL |
| 3 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | "No tables used" |
| 2 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | "No tables used" |
+----+-------------+-----------------+------------+--------+-----------------------+-----------------------+---------+-------------------------------+------+----------+------------------+