问题描述
我试图使用在多行上运行的SET
语句的update
子句来分配局部变量(实际上是两个)。好的,我正在执行ala MysqL。
drop table if exists stocks cascade;
create table stocks (
id serial,stock_available int,stock_locked int
);
insert into stocks(stock_available,stock_locked) values
(150,10),(150,20),0),(100,100),30),50),0);
create or replace function lock_all ()
returns int
language plpgsql as $$
declare
_amount int;
_total int;
begin
-- initialize accumulator
_total = 0;
-- update all the stocks table rows
update stocks
set _amount = stock_available,stock_locked = stock_locked + _amount,_total = _total + _amount;
-- returns the units locked
return _total;
end;
$$;
不幸的是,这不是Postgresql期望做这种事情的方式。
sql Error [42703]: ERROR: column "_amount" of relation "stocks" does not exist
Where: PL/pgsql function lock_all() line 10 at sql statement
这只是一个简单的示例,它说明了对update
句子中更新的事物进行计数/求和的实际问题。我相信这个特定示例可能有一些技巧或方法,但是我对这种情况下的通用解决方案感兴趣,因为这种情况下必须计算累加器。
有什么主意吗?
编辑
按照@GMB的建议,我链接了3个ctes
create or replace function lock_all3 ()
returns int
language sql as $$
with
cte1 as (
select
sum(stock_locked)::int as initially_locked
from
stocks
),cte2 as (
update
stocks
set
stock_locked = stock_locked + stock_available,stock_available = 0
returning
0 as dummy
),cte3 as (
select
sum(stock_locked)::int as finally_locked
from
stocks
)
select
(cte3.finally_locked - initially_locked - dummy)
from
cte1,cte2,cte3;
$$;
这应该起作用,但是结果值表明两个选择都在表stocks
的初始值上执行,因为两者之差为0。
select lock_all3();
lock_all3|
---------|
0|
但是,由于最终情况表明所有可用库存都已锁定,因此执行了cte2。
select * from stocks;
id|stock_available|stock_locked|
--|---------------|------------|
1| 0| 160|
2| 0| 170|
3| 0| 150|
4| 0| 100|
5| 0| 200|
6| 0| 130|
7| 0| 100|
8| 0| 150|
9| 0| 100|
在这种近似中,肯定还有些问题。
解决方法
我认为这样的结构不能在Postgres中工作;即使在MySQL中,也无法(或至少安全)以这种方式使用变量。
我认为我了解您要在执行更新之前跟踪可用的总库存。仅使用两个不同的查询可能会更简单:
select sum(stock_available) total from stocks returning total into _total;
update stocks set stock_locked = stock_locked + stock_available;
如果要避免竞争条件,可以将其包装在事务中,或将其写为单个语句:
with cte as (update stocks set stock_locked = stock_locked + stock_available)
select sum(stock_available) total from stocks returning total into _total;
,
我认为诀窍是在更新之前 计算总数。
仅使用SQL
DROP TABLE x;
SELECT sum(stock_available) as total_moved
INTO TEMP TABLE x
FROM stocks as total_moved;
UPDATE stocks
SET stock_locked = stock_available + stock_locked,stock_available = 0;
SELECT * from x;
total_moved|
-----------|
1050|
使用存储过程
create or replace function lock_all ()
returns int
language plpgsql as $$
declare
_total int;
begin
--calculate total before update
SELECT sum(stock_available)
INTO _total
FROM stocks;
UPDATE stocks
SET stock_locked = stock_locked + stock_available,stock_available = 0;
return _total;
end;
$$;
从lock_all中选择*;
lock_all|
--------|
1050|
从股票中选择*;
id|stock_available|stock_locked|
--|---------------|------------|
1| 0| 160|
2| 0| 170|
3| 0| 150|
4| 0| 100|
5| 0| 200|
6| 0| 130|
7| 0| 100|
8| 0| 150|
9| 0| 100|
,
这是一个肮脏解决方案,它执行的updates
与表stocks
中的行数一样多。它有效,但这是我试图避免的解决方案。
create or replace function lock_all ()
returns int
language plpgsql as $$
declare
_amount int;
_total int;
r record;
begin
-- initialize counter
_total = 0;
-- select stocks rows
for r in (
select * from stocks
)
loop
_amount = r.stock_available;
-- update the stock_fulfilled column in of_user_order_line_supply
update stocks
set stock_locked = stock_locked + _amount
where id = r.id;
_total = _total + _amount;
end loop;
return _total;
end;
$$;
select lock_all();
select * from stocks;
lock_all|
--------|
1050|