Re: Command Triggers

Excerpts from Robert Haas's message of lun dic 12 13:32:45 -0300 2011:
> On Sun, Dec 11, 2011 at 1:55 PM, Dimitri Fontaine
> <dimitri(at)2ndquadrant(dot)fr> wrote:
> > Andres Freund <andres(at)anarazel(dot)de> writes:
> >> Hm. I just noticed a relatively big hole in the patch: The handling of
> >> deletion of dependent objects currently is nonexistant because they don't go
> >> through ProcessUtility...
> >
> > That's the reason why we're talking about “command triggers” rather than
> > “DDL triggers”.  We don't intend to fire the triggers at each DDL
> > operation happening on the server, but for each command.
> >
> > This restriction still allows us to provide a very useful feature when
> > checked against the main use cases we target here. Those are auditing,
> > and replication (the replay will also CASCADEs), and a generic enough
> > SUDO facility (because the trigger function can well be SECURITY
> > DEFINER).
>
> I haven't yet thought about your specific proposal here in enough to
> have a fully-formed opinion, but I am a little nervous that this may
> turn out to be one of those cases where the obvious API ends up
> working less well than might have been supposed. For example,
> consider the "auditing" use case, and suppose that the user wants to
> log a message (to a table, or some other separate logging facility)
> every time someone creates an index. In order to do that, they're
> going to need to trap not only CREATE INDEX but also CREATE TABLE and
> ALTER CONSTRAINT,

Yeah. I remember mentioning the ability of CREATE SCHEMA to embed all
sort of object creation commands in a single top-level command, and
being handwaved away by Jan. "Nobody knows about that", I was told.
However, if this is to work in a robust way, these things should not be
ignored. In particular, firing triggers on each top-level command _is_
going to get some objects ignored; the people writing the triggers _are_
going to forget to handle CREATE SCHEMA properly most of the time.

--
Álvaro Herrera <alvherre(at)commandprompt(dot)com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support