PostgreSQL分组滚动平均值（用于多个标签）

我在网上找不到一个明确的例子

我想要一个基于此数据的过去两天的移动平均值：

create table expenses as (
  select 'food' as expense, 5.0 as cost, current_date as date
  union select 'food', 5.0, current_date - 1
  union select 'food', 4.0, current_date - 2
  union select 'food', 4.0, current_date - 3
  union select 'food', 3.0, current_date - 4
  union select 'food', 3.0, current_date - 5
  union select 'entertainment', 9.0, current_date
  union select 'entertainment', 9.0, current_date - 1
  union select 'entertainment', 8.0, current_date - 2
  union select 'entertainment', 8.0, current_date - 3
  union select 'entertainment', 7.0, current_date - 4
  union select 'entertainment', 7.0, current_date - 5
)

---

这是我提出的解决方案

select
 expense,
 date,
 cost,
 avg(cost) over
  (partition by expense order by date rows 2 preceding) as rolling_avg_cost
from expenses

结果如下：

expense       date                                cost rolling_avg_cost
entertainment Thursday, March 23, 2017 12:00 AM   7    7
entertainment Friday, March 24, 2017 12:00 AM     7    7
entertainment Saturday, March 25, 2017 12:00 AM   8    7.3
entertainment Sunday, March 26, 2017 12:00 AM     8    7.6
entertainment Monday, March 27, 2017 12:00 AM     9    8.3
entertainment Tuesday, March 28, 2017 12:00 AM    9    8.6
food          Thursday, March 23, 2017 12:00 AM   3    3
food          Friday, March 24, 2017 12:00 AM     3    3
food          Saturday, March 25, 2017 12:00 AM   4    3.3
food          Sunday, March 26, 2017 12:00 AM     4    3.6
food          Monday, March 27, 2017 12:00 AM     5    4.3
food          Tuesday, March 28, 2017 12:00 AM    5    4.6

可以看出，滚动平均值的窗口为3天，包括当前行（即当前行加上前两行，全部除以3）