Postgres在转换时返回函数错误/失败时的空值

Question

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

I am attempting to perform a select with a cast as follows:

select id, c1, c1::timestamp as c2 from a;

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:

ERROR: date/time field value out of range: "24-04-89 2:33 am"
HINT: Perhaps you need a different "datestyle" setting.

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)

---

EDIT:
Also, is there a generic way to implement this? ie: (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 eg: 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;

or

select id, c1, set_null_on_error(to_number(c1, '99')) as c2 from a;

Answer 1

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;

---

Trying to define a generic function.

Assume that you defined a function set_null_on_error(anyelement) . Calling

select set_null_on_error('foo'::timestamp);

raises error before the function is executed.

You can try something like this:

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']);

In my opinion such a solution is too complicated, quite inconvenient to use and generally might turn out to generate problems hard to debug.