Re: Using regoper type with OPERATOR()

On 6 October 2011 12:43, Gavin Flower <GavinFlower@archidevsys.co.nz> wrote:
> On 05/10/11 18:42, Tony Theodore wrote:
[...]
>> so I could use a query like:
>>
>> SELECT price OPERATOR(disc_oper::regoper) disc AS disc_amount FROM
>> discounts
>>
>> This doesn't work however, and I'm not sure why. I think I'm missing
>> something simple since:
[...]
>>
> I suugests:
> (1) using the 'money' type instead of float
> (2) using an enum instedd of regoper
>
[...]
>
> SELECT
>    i.name,
>
>    CASE
>        WHEN d.type = 'amount'::discount_type THEN i.price - d.amount
>        WHEN d.type = 'fraction'::discount_type THEN i.price * d.fraction
>        ELSE i.price
>    END AS "displayed price"
> FROM
>    item i LEFT JOIN discount d ON (i.discount_id = d.id)
> ORDER BY
>    i.name

Hi Gavin, thanks for the suggestion - after thinking about it some
more, what I'm actually trying to do is avoid predefined CASE
statements (and enums). More generally, I'm looking for a general way
to do function/operator lookups so it's possible to specify/modify the
logic of certain calculations easily.

I found the "Executing Dynamic Commands" docs and a function such as:

CREATE OR REPLACE FUNCTION var_op(left_ double precision, right_
double precision, operator_ text)
  RETURNS double precision AS
$$
DECLARE result double precision;
BEGIN
    EXECUTE 'SELECT $1 OPERATOR(' || operator_::regoperator::regoper || ') $2'
    INTO result
    USING left_, right_;
    RETURN result;
END;
$$
  LANGUAGE plpgsql;

will achieve the result I'm after - but I'm not sure if this is a good idea.

Thanks,

Tony