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Re: Vote on SET in aborted transaction

From: Thomas Lockhart <lockhart(at)fourpalms(dot)org>
To: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Vote on SET in aborted transaction
Date: 2002-04-24 14:23:27
Message-ID: 3CC6BFDF.BCB12986@fourpalms.org (view raw or flat)
Thread:
Lists: pgsql-hackers
> OK, would people please vote on how to handle SET in an aborted
> transaction?
> at the end, should 'x' equal:
>         1 - All SETs are rolled back in aborted transaction
>         2 - SETs are ignored after transaction abort
>         3 - All SETs are honored in aborted transaction
>         ? - Have SETs vary in behavior depending on variable

I'll vote for "?", if for no other reason that you are proposing taking
away a huge chunk of "language space" by apriori disallowing out of band
behaviors for anything starting with "SET". I think that is likely
Hiroshi's concern also.

If we can fit all current "SET" behaviors into a transaction model, then
I'm not against that (though we should review the list of attributes
which *are* currently affected before settling on this). afaik we have
not reviewed current behaviors and have not thought through the "what
if's" that some soft of premature policy decision might constrain in the
future.

Let me give you some examples. We might someday have nested
transactions, or transactions which can be resumed from the point of
failure. We *might* want to be able to affect recovery behaviors, and we
*might* want to do so with something like

begin;
update foo...
update bar...
<last update fails>
set blah to blah
update baz...
update bar...
<last update now succeeds>
end;

Now we currently *don't* support this behavior, but istm that we
shouldn't preclude it in the language by forcing some blanket "all SET
statements will be transaction aware".

What language elements would you propose to cover the out of band cases
if you *do* disallow "SET" in that context? If you don't have a
candidate, I'd be even more reluctant to go along with the results of
some arbitrary vote which is done in a narrow context.

And btw, if we *are* going to put transaction semantics on all of our
global variables (which is the context for starting this "SET"
discussion, right? Is that really the context we are still in, even
though you have phrased a much more general statement above?) then let's
have the discussion on *HOW* we are going to accomplish that *BEFORE*
deciding to make a semantic constraint on our language support.

Hmm, if we are going to use transaction semantics, then we should
consider using our existing transaction mechanisms, and if we use our
existing transaction mechanisms we should consider pushing these global
variables into tables or in memory tables a la "temp tables". We get the
transaction semantics for free, with the cost of value lookup at the
beginning of a transaction or statement (not sure what we can get away
with here).

If we are *not* going to use those existing mechanisms, then what
mechanism *are* we going to use? Some sort of "abort hook" mechanism to
allow SET to register things to be rolled back?

If we end up making changes and increasing constraints, then we should
also expect some increased functionality as part of the scheme,
specifically "SET extensibility". We should allow (future) packages to
define their parameters and allow SET to help.

Just some thoughts...

                       - Thomas

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Subject: Re: Inefficient handling of LO-restore + Patch
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