Re: [HACKERS] proposal: session server side variables

2016-12-29 14:25 GMT+01:00 Fabien COELHO <coelho@cri.ensmp.fr>:

I newer talked about persistent data. I talked about persistent metadata.

Sure, I finally understood that detail. Now if I hear "persistent variable", I by default understand that both metadata and data are persistent... It requires some effort to understand the subtelty.

I really don't propose any possible substitution of tables (relations). I newer did it.

Sure.

The used terminology is not 100% clean and natural - maybe better name is
"global temporary unshared untransactional unrelational storage" -

Hmmm. Too long:-) But these properties need to be spelled out.

[...] I don't see any sense to have two similar storages or two redundant access methods - not in PostgreSQL level.

Then say so in the wiki in the cons.

Personnaly, I'm not sure. Maybe having a clean way of declaring a one-row "singleton" table enforced by postgresql would be enough.

There is a singleton table :)

create table foo(x integer unique not null default 1 check(x = 1), y  integer);

insert into foo(y) values(100);

analyze foo;

The storage is not important and is not interesting - any different behave for persistent objects different than MVCC can be big surprise for users. 

What is interesting are getter functions - they can be volatile or stable/immutable - what can be interesting, because then the value can be used by planner. 

For example - MySQL @var is volatile - can be changed in query - that's mean, you cannot use it as const for planner :( - the behave will be same (with same risks to performance) like using plpgsql variable in query.

With getter functions you can do bigger game.

Regards

Pavel

--
Fabien.