Postgresql 更新Postgres中的列只返回一个错误值（Postgres 9.5.1）

假设我有一个由以下内容创建的表：

create table test.sales
(
 customer text,
 purchased text,
 date_purchased date,
 rownumber integer,
 primary key (customer, date_purchased)
)
;

insert into test.sales
values
('kevin', 'books', '2017-01-01'::date, null),
('kevin', 'movies', '2017-01-02'::date, null),
('paul', 'books', '2017-01-05'::date, null),
('paul', 'movies', '2017-01-07'::date, null)

update test.sales as a
set (rownumber) =
(
select
  row_number() over (partition by customer order by date_purchased) as rownumber
from test.sales as b
-- These fields correspond to the primary keys of the table.
where a.customer = b.customer
  and a.date_purchased = b.date_purchased
)

此时，

rownumber

始终为

NULL

，我想将

rownumber

的值设置为rownumber（

row\u number（））（按客户订单按购买日期划分）。我的做法如下：

create table test.sales
(
 customer text,
 purchased text,
 date_purchased date,
 rownumber integer,
 primary key (customer, date_purchased)
)
;

insert into test.sales
values
('kevin', 'books', '2017-01-01'::date, null),
('kevin', 'movies', '2017-01-02'::date, null),
('paul', 'books', '2017-01-05'::date, null),
('paul', 'movies', '2017-01-07'::date, null)

update test.sales as a
set (rownumber) =
(
select
  row_number() over (partition by customer order by date_purchased) as rownumber
from test.sales as b
-- These fields correspond to the primary keys of the table.
where a.customer = b.customer
  and a.date_purchased = b.date_purchased
)

但出于某种原因，这种情况再次出现：

customer    purchased   date_purchased  rownumber
kevin        books         1/1/17          1
kevin        movies        1/2/17          1
paul         books         1/5/17          1
paul         movies        1/7/17          1

我期待着：

customer    purchased   date_purchased  rownumber
kevin        books         1/1/17          1
kevin        movies        1/2/17          2
paul         books         1/5/17          1
paul         movies        1/7/17          2

请注意，在实际结果中，
rownumber
始终为1。这是为什么？
您使用的是标量子查询。请改用相关子查询：


您正在使用标量子查询。请改用相关子查询：


有趣！我是否应该担心效率/可扩展性？我的理解是，相关子查询不能很好地处理大量行。请测试它。（至少结果是正确的，所以测试还是没用的…）有趣！我是否应该担心效率/可扩展性？我的理解是，相关子查询不能很好地处理大量行。请测试它。（至少结果是正确的，所以测试还是没用的…）