Question

我正在尝试将子查询的结果保存到表中。

select (b.close - a.close) AS difference from day_ohlcs AS a inner join day_ohlcs AS b ON a.id + 1 = b.id

子查询的速度很快，但是当我尝试将它放在更新语句中时，它需要永远，所以我知道我做错了。

UPDATE day_ohlcs
SET price_change = t.difference
FROM (select (b.close - a.close) AS difference
    FROM day_ohlcs AS a 
    inner join day_ohlcs AS b 
    ON a.id + 1 = b.id) 
AS t
WHERE day_ohlcs.instrument_id = 1;

我不理解什么？提前谢谢。

Answer 1

我认为这是因为您还没有指定要加入的day_ohlcs列，例如：

UPDATE day_ohlcs
SET price_change = t.difference
FROM (select a.id, (b.close - a.close) AS difference
    FROM day_ohlcs AS a 
    inner join day_ohlcs AS b 
    ON a.id + 1 = b.id) 
AS t
WHERE day_ohlcs.instrument_id = 1 and day_ohlcs = t.id

实际上看起来你可以在没有子查询的情况下做到这一点

update day_ohlcs as a set
   price_change = b.close - a.close
from day_ohlcs as b 
where b.id = a.id + 1 and a.instrument_id = 1

Answer 2

两个查询之间的语义非常不同。请注意，day_ohlcs出现两次而不是三次 - 后者会进行太多的计算，主要是计算整个表的差异。

您可以删除不需要的行为，如下所示：

UPDATE day_ohlcs as a
SET price_change = b.close - a.close
FROM day_ohlcs AS b
WHERE a.id + 1 = b.id and a.instrument_id = 1;

http://www.postgresql.org/docs/current/static/sql-update.html

如何使用子查询更新多行？

2 个答案: