在进行MySQL性能检查时，我们发现代码中进行了一些全表扫描。 我们创建了一个视图，该视图跨越了不同的表，这些表是主数据-详细记录。例如，销售和销售线。
在此主记录中，我们需要添加分页。因此，我们必须能够获取前10个销售（及其相应的销售线）。还要获取前10个销售，这些销售具有一个销售单价高于10 $的销售线。
因此，在这种情况下，我们创建了一个视图vw_sales，该视图将销售线和销售线连接在一起，并且我们能够满足这些要求。
可以在此处查看创建表
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;
视图在此处创建
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`);
我们添加了如下的分页
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
explain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（实例化视图）的限制，我们导致了一些全表扫描-table sale_test。 （参考How do I get MySQL to use an INDEX for view query?，Does MySQL view always do full table scan?）
似乎无法跨表创建覆盖索引，因此该选项也无法实现。
我们已经在视图内部添加了力索引，以提示正确的索引，但这不能解决问题。
如何以下降的方式解决这个问题？创建临时表不会引起很多开销吗？ 这似乎很简单，但是分页要求却使其变得非常困难。 我们正在使用mysql 5.7（AWS RDS），因此mysql 8.0中添加的窗口函数不是我们的选择。
仅供参考：在没有视图的情况下，我们看到索引正在使用中
查询：
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" |
+----+-------------+-----------------+------------+--------+-----------------------+-----------------------+---------+-------------------------------+------+----------+------------------+