转换时Postgres在函数错误/失败时返回空值 [英] Postgres return null values on function error/failure when casting
问题描述
我正在尝试将text
值转换为timestamp
值.
对于名为 a
的下表:
I am attempting to convert text
values to timestamp
values.
For the following table called a
:
id | c1
----+--------------------
1 | 03-03-2000
2 | 01-01-2000
3 | 12/4/1990
4 | 12 Sept 2011
5 | 12-1-1999 12:33:12
6 | 24-04-89 2:33 am
我正尝试通过 cast 执行select
,如下所示:
I am attempting to perform a select
with a cast as follows:
select id, c1,
c1::timestamp
as c2 from a;
如果只有前5行,这是正确的,但是对于c1
为24-04-89 2:33 am
的第6行,它将引发以下错误:
This works correctly if there were only the first 5 rows, but for the 6th row where c1
is 24-04-89 2:33 am
it throws the following error:
错误:日期/时间字段值超出范围:"24-04-89 2:33 am"
提示:也许您需要一个不同的日期样式"设置.
ERROR: date/time field value out of range: "24-04-89 2:33 am"
HINT: Perhaps you need a different "datestyle" setting.
我想要的是 null
,用于那些无法转换为时间戳的值,而不是命令完全失败.像这样:
What I want is null
for those values which cannot not be casted to timestamp instead of the command failing altogether. Like this:
id | c1 | c2
----+--------------------+---------------------
1 | 03-03-2000 | 2000-03-03 00:00:00
2 | 01-01-2000 | 2000-01-01 00:00:00
3 | 12/4/1990 | 1990-12-04 00:00:00
4 | 12 Sept 2011 | 2011-09-12 00:00:00
5 | 12-1-1999 12:33:12 | 1999-12-01 12:33:12
6 | 24-04-89 2:33 am | (null)
(6 rows)
另外,有没有通用的方法来实现这一点?即:(基于 klin的答案)一个plpgsql包装函数,如果包装了该函数,则将其值设置为null
周围抛出一个错误.
例如:函数set_null_on_error
可以像这样使用:
Also, is there a generic way to implement this? i.e.: (based on klin's answer) a plpgsql wrapper function that sets the value to null
if the function it is wrapped around throws an error.
For e.g.: a function set_null_on_error
that can be used like this:
select id, c1,
set_null_on_error(c1::timestamp)
as c2 from a;
或
select id, c1,
set_null_on_error(to_number(c1, '99'))
as c2 from a;
推荐答案
这可以通过在plpgsql函数中捕获异常来完成.
This can be done by trapping an exception in a plpgsql function.
create or replace function my_to_timestamp(arg text)
returns timestamp language plpgsql
as $$
begin
begin
return arg::timestamp;
exception when others then
return null;
end;
end $$;
select id, c1, my_to_timestamp(c1) as c2 from a;
尝试定义泛型函数.
Trying to define a generic function.
假设您定义了一个函数set_null_on_error(anyelement)
.呼叫
Assume that you defined a function set_null_on_error(anyelement)
. Calling
select set_null_on_error('foo'::timestamp);
在执行该功能之前引发错误.
raises error before the function is executed.
您可以尝试以下操作:
create or replace function set_null_on_error(kind text, args anyarray)
returns anyelement language plpgsql
as $$
begin
begin
if kind = 'timestamp' then
return args[1]::timestamp;
elseif kind = 'number' then
return to_number(args[1], args[2]);
end if;
exception when others then
return null;
end;
end; $$;
select set_null_on_error('timestamp', array['2014-01-01']);
select set_null_on_error('number', array['1.22444', '9999D99']);
我认为这样的解决方案太复杂了,使用起来非常不方便,并且通常可能会产生难以调试的问题.
In my opinion such a solution is too complicated, quite inconvenient to use and generally might turn out to generate problems hard to debug.
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