如何在Redshift中查找每天局部最大值的總和？

Question

我有上升的時間戳一些數值數據如下：

amount | received_at 
_______|______________ 
30  | 2016-11-18 10:21:35 AM 
60  | 2016-11-18 10:22:05 AM 
90  | 2016-11-18 10:22:35 AM 
120 | 2016-11-18 10:23:05 AM 
150 | 2016-11-18 10:23:35 AM 
160 | 2016-11-18 10:24:05 AM 
0  | 2016-11-18 10:26:00 AM 
20  | 2016-11-18 10:26:20 AM 
40  | 2016-11-18 10:26:40 AM 
55  | 2016-11-18 10:26:50 AM 

我需要增加160和55得到的總數。這不僅限於兩個數字，但對於給定的一組列，可以是每天需要添加的多個此類最大值。有沒有一個簡單的（足夠的）解決方案呢？

我使用Redshift來計算這個數字。

Answer 1

假設你的表是create table t(amount int, received_at timestamptz);

1）存儲功能：

create function foo() returns setof t language plpgsql immutable as $$ 
declare 
    r t; 
    c t; 
begin 
    r := null; 
    for c in (select * from t order by received_at) loop 
    if r is null or r.amount < c.amount then 
     r := c; 
    else 
     return next r; 
     r := c; 
    end if; 
    end loop; 
    if r is not null then 
    return next r; 
    end if; 
end $$; 

select * from foo(); 

2）窗口函數：

with cte as (
    select 
    amount, 
    received_at, 
    case 
     when coalesce(lead(amount) over (order by received_at), 0) < amount then 1 
     else 0 
    end as flag 
    from t) 
select amount, received_at from cte where flag = 1; 

結果：

╔════════╤════════════════════════╗ 
║ amount │  received_at  ║ 
╠════════╪════════════════════════╣ 
║ 160 │ 2016-11-18 10:24:05+02 ║ 
║  55 │ 2016-11-18 10:26:50+02 ║ 
╚════════╧════════════════════════╝ 

聲明：我不確定您想在當天穿越什麼。

Answer 2

下面是一個方法：

select sum(amount) 
from (select t.*, 
      row_number() over (partition by date_trunc('day', received_at) order by amount desc) as seqnum 
     from t 
    ) t 
where seqnum = 1;