Re: Freezing localtimestamp and other time function on some value

On Tue, Apr 12, 2016 at 10:14 AM, Alex Ignatov <a(dot)ignatov(at)postgrespro(dot)ru>
wrote:

>
>
> On 12.04.2016 19:45, David G. Johnston wrote:
>
> On Tue, Apr 12, 2016 at 8:37 AM, Alex Ignatov < <a(dot)ignatov(at)postgrespro(dot)ru>
> a(dot)ignatov(at)postgrespro(dot)ru> wrote:
>
>>
>> On 12.04.2016 18:01, Adrian Klaver wrote:
>>
>>>
>>> >>I do it by having the date be one of the function arguments and have
>> the default be something like current_date. When I test I supply a date to
>> override the default. This allows for testing the various scenarios by
>> changing the supplied date.
>>
>> With that approach you have to say application programmer - 'Hey dude,
>> please edit this piece of code for my purpose and after that rollback it'.
>> I think that it is unacceptable in large project...
>
>
> ​ CREATE FUNCTION do_some_date_based_stuff(reference_date date,
> other_args) [...]
>
> CREATE FUNCTION production_wrapper_for_above(other_args) [...]
> AS $$
> SELECT do_some_date_based_stuff(now(), other_args);
> $$ ​;
>
> Easy to test do_some_date_based_stuff since it has fewer if any external
> dependencies. Shouldn't need to test the wrapper that simply calls the
> "do_some..." with a default value of the current date.
>
> You might be able to define an appropriate function signature that avoids
> having to write the wrapper though regardless there is no need to have a
> different environment for testing versus production if approached in this
> manner. You just need to decide on the most desirable way to make it work.
>
> David J.
>
>
> I know that we can always write some wrappers etc, etc.
> This approach would failed if your do_some_date_based_stuff have no date
> args and contains calls say to now()(or other time function what possible
> can have fix value ) inside it.
>
>
This makes no sense and I'm not sure how to explain it better. The point
is for any functions where you think you need to "freeze/specify" time​ you
write the procedure function so that is always uses a caller-specified
reference time. If you want to provide access to the logic without
requiring the caller to supply a reference time then do so. Or just make
callers supply the time they care about. But you'd never put "now()" into
such a function - wherever you would use "now()" you'd use the argument
date instead. IOW, we don't support the feature in question so you have to
modify code if you want similar functionality in PostgreSQL.

Also wrappers lead to multiple code base,yours client side code needs to
> know what function we should use - test or production. Also with your
> approach application server needs to know its working mode test / prod
>
> You always should keep in mind that your application may run in test mode
> (future/past time) and maintain this code. While with my proposal you can
> always use some time function(now or localtimestamp or whatever) which you
> can freeze at anytime on DB level, not operation system(using some 3rd
> libs) or application(using wrappers and other hacks).
>
>
​My proposal has nothing to do with test versus development. Both are
always present and the choice of which function to call​

​is up to the code needing to perform the work.
​

I'm not sure what you are looking for here. PostgreSQL doesn't have this
particular feature. You can either convince others to write it, write it
yourself and work to get it committed, or learn a new approach to solving
the problem that you describe. My intent is to aiding effort toward the
third option.

David J.