Re: proposal: session server side variables

2017-01-03 20:56 GMT+01:00 Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>:

>
> Hello,
>
> Probably there is not big difference between RESET and UNDO in complexity
>> of implementation. You have to do partial implementation of MVCC. No
>> simple
>> code.
>>
>
> I think so; yes; indeed.
>
> Also note that user-defined GUCs already implements the transactional
>>> property, so probably the mecanism is already available and can be
>>> reused.
>>>
>>
>> GUC are stack based - the value doesn't depends if transaction was
>> successful or not.
>>
>
> Hmmm... this looks transactional to me:
>
> SELECT set_config('x.x', 'before', FALSE); -- 'before'
> BEGIN;
> SELECT set_config('x.x', 'within', FALSE); -- 'within'
> ROLLBACK;
> SELECT current_setting('x.x'); -- 'before'
> BEGIN;
> SELECT set_config('x.x', 'inside', FALSE); -- 'inside'
> COMMIT;
> SELECT current_setting('x.x'); -- 'inside'
>
> I would say the stack is needed for SAVEPOINT:
>
> SELECT set_config('x.x', 'before', FALSE); -- 'before'
> BEGIN;
> SELECT set_config('x.x', 'within', FALSE); -- 'within'
> SAVEPOINT within;
> SELECT set_config('x.x', 'inside', FALSE); -- 'inside'
> SELECT current_setting('x.x'); -- 'inside'
> ROLLBACK TO SAVEPOINT within;
> SELECT current_setting('x.x'); -- 'within'
> SELECT set_config('x.x', 'further', FALSE); -- 'further'
> ROLLBACK;
> SELECT current_setting('x.x'); -- 'before'
>
> So basically the use case needs GUCs with some access control. Or just
> role-private GUCs and some access function tricks would do as well for the
> use case. At least it is probably much easier to add privacy to gucs than
> to (re)implement permissions and MVCC on some session variables. And it
> would be nice if GUCs could be typed as well...

With respect, I don't share your opinion - it is not enough for usage like
package variables - there usually should not to use any dependency on
transactions.

More it is dynamic - it should be hard inconsistency to implement CREATE or
DECLARE statement for GUC. So it is out my proposal (and my goal).

Regards

Pavel

>
>
> --
> Fabien.
>