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Re: Command Triggers patch v18

From: Dimitri Fontaine <dimitri(at)2ndQuadrant(dot)fr>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Dimitri Fontaine <dimitri(at)2ndquadrant(dot)fr>, Andres Freund <andres(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Command Triggers patch v18
Date: 2012-03-30 15:19:23
Message-ID: m2sjgqjev8.fsf@2ndQuadrant.fr (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> Oh, right: I remember that now.  I still think it's a bad way to do
> it, because the trigger potentially has a lot of work to do to
> reconstruct a working command string, and it still ends up getting
> executed by the wrong user.  For CREATE EXTENSION it's not that bad,

That's true, I'm only kind of saying that the INSTEAD OF keyword still
makes sense (instead of security_checks). I agree that the feature is
simpler to use the way to propose it.

>>   context = 'DROP TYPE'
>>   tag = 'DROP FUNCTION'
>
> I'm not sure I understand how you're using the words "context" and

Yeah context is not explicit, we could call that "toplevel": the command
tag of the command that the user typed. When toplevel is null, the event
trigger is fired on a command the user sent, when it's not null, the
trigger is fired on some inner command operation.

> "tag".  I think for a drop trigger I would want the function to
> receive this information: type of object dropped, OID of object
> dropped, column number in the case of a column drop, flag indicating
> whether it's a toplevel drop or a cascaded drop.  I wouldn't object to
> also making the currently-in-context toplevel command tag available,
> but I think most drop triggers wouldn't really care, so I wouldn't
> personally spend much implementation effort on it if it turns out to
> be hard.

I'm not sure it would be hard as I'm only seeing a single depth possible
here, so a single per-backend string static variable would do.

> But in general, I don't really know what a "proper" subcommand is or
> why some subcommands should be more proper than others, or why we
> should even be concerned about whether something is a subcommand at
> all.  I think it's fine and useful to have triggers that fire in those
> kinds of places, but I don't see why we should limit ourselves to
> that.  For applications like replication, auditing, and enhanced
> security, the parse tree and subcommand/non-subcommand status of a
> particular operation are irrelevant.  What you need is an exact

Not really. When replicating you could perfectly say that you only
replicate the toplevel DROP because the replica will also do the cascade
dance and you might have decided not to replicate all related objects on
the other side.

The information you need really want not to miss is when only the
cascaded object is part of the replication, not the main one. That was
not covered by my previous patch but now we have a way to cover it.

> description of the operation that got performed (e.g. the default on
> table X column Y got dropped); you might be able to reverse-engineer
> that from the parse tree, but it's much better to have the system pass
> you the information you need more directly.  Certainly, there are
> cases where you might want to have the parse tree, or even the raw
> command text, available, but I'm not even convinced that that those
> cases will be the most commonly used ones unless, of course, they're
> the only ones we offer, in which case everyone will go down that path
> by necessity.

There are far too many variants and cases of our command to be able to
extract their parameters in a flat way (a bunch of variables compared to
a nested description ala json or xml), and I don't think such a flat
representation is going to be much better than the parse tree.

Now, we will later be able to offer a normalized rewritten command
string from the parse tree to the use, but I don't see us adding support
for that from cascaded drops, one other reason why I like to expose them
as sub commands.

> Again, I'm not understanding the distinction between toplevel events
> and sub-events.  I don't see any need for such a distinction.  I think
> there are just events, and some of them happen at command start/end
> and others happen somewhere in the middle.  As long as it's a safe and
> useful place to fire a trigger, who cares?

I guess you're head is too heavily in the code side of things as opposed
to the SQL user view point. Maybe my attempt to conciliate both views is
not appropriate, but I really do think it is.

>> Given the scope of this mini expression language, we can easily bypass
>> calling the executor in v1 here, and reconsider later if we want to
>> allow calling a UDF in the WHEN clause… I don't think it's an easy
>> feature to add in, though.
>
> Or a necessary one.  AFAICS, the main benefit of WHEN clauses on

Exactly.

> regular triggers is that you can prevent the AFTER trigger queue from
> getting huge, and maybe a save a little on the cost of invoking a
> trigger function just to exit again.  But neither of those should be
> relevant here - nobody does that much DDL, and anybody writing command
> triggers should understand that this is advanced magic not intended
> for beginners.  Wizards below level 10 need not apply.

Here it's only a facility to manage your event trigger code
organization, and I insisted in having it in the syntax because in the
event trigger grammar I don't see another place where to stuff which
command you're targeting. The command tag certainly can not be
considered as another kind of event.

As much as I like the event timing idea and appreciate that it nicely
solves some problem that were hairy with the previous BEFORE/AFTER too
simple syntax, I want to keep it sane.

An event timing is a generic named place in the code where we “hook” the
execution of one or several use defined function. This place is generic
because a whole series of command are providing the hooking place, such
as command_start, command_end, permissions_check, consistency_check, etc
etc.

I foresee a matrix in the documentation in the form of


            timing | command_start | pcheck | check | command_end | …
  command /        |               |        |       |             | …
  -----------------+---------------+--------+-------+-------------+----
   GRANT           |     ✔        |  ✘     |  ✘   |  ✔
   CREATE TABLE    |     ✔        |  ✔     |  ✔   |  ✔
   ALTER TABLE     |     ✔        |  ✔     |  ✔   |  ✔
   …

I don't think GRANT will be there in 9.2 by the way, but it helps seeing
commands that won't support all of events to express the idea, I guess.

> BTW, in reference to your other email, the reason I suggested
> ddl_command_start rather than just command_start is because we already
> know that there are cases here we'll never be able to handle, like
> before-begin.  I think it's better to be explicit about what the scope

I agree with the fact that any given command will not be able to support
any given timing spec, what we need is document them. That said, I don't
see any gain in spelling command_start differently depending on whether
we are applying to some kind of command or some other.

> of the feature is (i.e. DDL) rather than just calling it command_start
> and, oh, by the way, the commands we either don't know how to support
> or didn't get around to supporting yet aren't included.  The latter
> approach is basically a guaranteed compatibility break every time we
> get around to adding a few more commands, and I'm not keen on that.

I don't buy that argument. What would be the difference between adding
support for DCL commands and adding a new DDL in term of compatibility
breakage for event triggers that are firing on all commands? In both
cases they will get called on non anticipated events, the fact that one
event is called ddl_command_start and the other dcl_command_start does
not appear to me to be relevant in the least. I might be missing
something though.

Regards,
--
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support

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