Re: [v9.3] OAT_POST_ALTER object access hooks

2012/12/11 Robert Haas <robertmhaas(at)gmail(dot)com>:
> On Mon, Dec 3, 2012 at 9:59 AM, Kohei KaiGai <kaigai(at)kaigai(dot)gr(dot)jp> wrote:
>> As we discussed before, it is hard to determine which attributes shall
>> be informed to extension via object_access_hook, so the proposed
>> post-alter hook (that allows to compare older and newer versions)
>> works fine on 99% cases.
>> However, I'm inclined to handle SET TABLESPACE as an exception
>> of this scenario. For example, an idea is to define OAT_PREP_ALTER
>> event additionally, but only invoked very limited cases that takes
>> unignorable side-effects until system catalog updates.
>> For consistency of hook, OAT_POST_ALTER event may also ought
>> to be invoked just after catalog updates of pg_class->reltablespace,
>> but is_internal=true.
>>
>> How about your opinion?
>
> I don't really like it - I think POST should mean POST. You can
> define some other hook that gets called somewhere else, but after
> means after. There are other plausible uses of these hooks besides
> sepgsql; for example, logging the completion time of an action.
> Putting the hooks in the "wrong" places because that happens to be
> more convenient for sepgsql is going to render them useless for any
> other purpose. Maybe nobody else will use them anyway, but I'd rather
> not render it impossible before we get off the ground.
>
I have to admit "PREP" hook is problematic to determine which information
should be delivered, and which should not, as we had discussed before.
Yes, a hook convenient for sepgsql, might be unconvenient for others.

So, which alternatives do we have? I can list up a few ideas.

1) Putting POST_ALTER hook after existing whole table rewrite.
good: no need to have significant code change
bad: it takes heavy i/o prior to hook invocation

2) Updating reltablespace of pg_class prior to whole table rewrite,
and put POST_ALTER hook just after catalog update.
good: well fit with assumption of POST hook.
bad: it takes significant code changes on table rewriting

3) Using ProcessUtility_hook to track AKTER TABLE ... SET
TABLESPACE as an exception.
good: no need to have significant code change
bad: sepgsql also have to have analyzer of ALTER TABLE
commands.

I think (2) is the best way, if feasible. I need to investigate whether
the relevant code allows to implement catalog updates prior to
whole table rewriting.
However, (1) is also not so bad in a short term, as a first step towards
the idea (2) that takes additional code updates.
Of course, I can implement with (3), but not inclined with this idea.

Is there other idea?

Thanks,
--
KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>