Re: [HACKERS] proposal: schema variables

2018-09-07 14:34 GMT+02:00 Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>:

>
> Hello Pavel,
>
> here is updated patch - I wrote some transactional support
>>
>> I am not sure how these new features are understandable and if these
>> features does it better or not.
>>
>
> There are possibility to reset to default value when
>>
>> a) any transaction is finished - the scope of value is limited by
>> transaction
>>
>> CREATE VARIABLE foo int ON TRANSACTION END RESET;
>>
>
> With this option I understand that it is a "within a transactionnal"
> variable, i.e. when the transaction ends, whether commit or rollback, the
> variable is reset to a default variable. It is not really a "session"
> variable anymore, each transaction has its own value.
>

yes, the correct name should be "schema variable with transaction scope". I
think it can be useful like short life global variable. These variables can
works like transaction caches.

> -- begin session
> -- foo has default value, eg NULL
> BEGIN;
> LET foo = 1;
> COMMIT/ROLLBACK;
> -- foo has default value again, NULL
>
> b) when transaction finished by rollback
>>
>> CREATE VARIABLE foo int ON ROLLBACK RESET
>>
>
> That is a little bit safer and you are back to a SESSION-scope variable,
> which is reset to the default value if the (any) transaction fails?
>
> -- begin session
> -- foo has default value, eg NULL
> BEGIN;
> LET foo = 1;
> COMMIT;
> -- foo has value 1
> BEGIN;
> -- foo has value 1...
> ROLLBACK;
> -- foo has value NULL
>
> c) A more logical (from a transactional point of view - but not necessary
> simple to implement, I do not know) feature/variant would be to reset the
> value to the one it had at the beginning of the transaction, which is not
> necessarily the default.
>
> -- begin session
> -- foo has default value, eg NULL
> BEGIN;
> LET foo = 1;
> COMMIT;
> -- foo has value 1
> BEGIN;
> LET foo = 2; (*)
> -- foo has value 2
> ROLLBACK;
> -- foo has value 1 back, change (*) has been reverted
>
> Now, when I am thinking about it, the @b is simple, but not too practical -
>> when some fails, then we lost a value (any transaction inside session can
>> fails).
>>
>
> Indeed.
>
> The @a has sense - the behave is global value (what is not possible
>> in Postgres now), but this value is destroyed by any unhandled exceptions,
>> and it cleaned on transaction end. The @b is just for information and for
>> discussion, but I'll remove it - because it is obscure.
>>
>
> Indeed.
>
> The open question is syntax. PostgreSQL has already ON COMMIT xxx . It is
>> little bit unclean, because it has semantic "on transaction end", but if I
>> didn't implement @b, then ON COMMIT syntax can be used.
>>
>
> I was more arguing on the third (c) option, i.e. on rollback the value is
> reverted to its value at the beginning of the rollbacked transaction.
>

> At the minimum, ISTM that option (b) is enough to implement the audit
> pattern, but it would mean that any session which has a rollback, for any
> reason (deadlock, serialization...), would have to be reinitialized, which
> would be a drawback.
>
> The to options could be non-transactional session variables "ON ROLLBACK
> DO NOT RESET/DO NOTHING", and somehow transactional session variables "ON
> ROLLBACK RESET TO DEFAULT" (b) or "ON ROLLBACK RESET TO INITIAL" (c).
>

@b is hardly understandable for not trained people, because any rollback in
session does reset. But people expecting @c, or some near @c.

I understand so you talked about @c. Now I think so it is possible to
implement, but it is not trivial. The transactional behave have to
calculate not only with transactions, but with SAVEPOINTS and ROLLBACK TO
savepoints. On second hand, the implementation will be relatively compact.

I'll hold it in my memory, but there are harder issues (support for
parallelism).

Regards

Pavel

> --
> Fabien.
>
>