Re: Add new protocol message to change GUCs for usage with future protocol-only GUCs

Jelte Fennema-Nio <me(at)jeltef(dot)nl> writes:
> Currently the only way to set GUCs from a client is by using SET
> commands or set them in the StartupMessage. I think it would be very
> useful to be able to change settings using a message at the protocol
> level. For the following reasons:

> 1. Protocol messages are much easier to inspect for connection poolers
> than queries

Unless you somehow forbid clients from setting GUCs in the old way,
exactly how will that help a pooler?

> 2. It paves the way for GUCs that can only be set using a protocol
> message (and not using SET).

This is assuming facts not in evidence. Why would we want such a thing?

> 3. Being able to change GUCs while in an aborted transaction.

I'm really dubious that that's sane. How will it interact with, for
example, changes to the same GUC done in the now-failed transaction?
Or for that matter, changes that happen later in the current
transaction? It seems like you can't even think about this unless
you deem GUC changes made this way to be non-transactional, which
seems very wart-y and full of consistency problems.

> 4. Have an easy way to use the value contained in a ParameterStatus
> message as the value for a GUC

I agree that GUC_LIST_QUOTE is a mess, but "I'm too lazy to deal
with that" seems like a rather poor argument for instead expending
the amount of labor involved in a protocol change.

On the whole, this feels like you are trying to force some things
into the GUC model that should not be there. I do perceive that
there are things that could be protocol-level variables, but
trying to say they are a kind of GUC seems like it will create
more problems than it solves.

regards, tom lane