From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@2ndquadrant.com>
cc: Dimitri Fontaine <dimitri@2ndQuadrant.fr>,
	Simon Riggs <simon@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Event Triggers: adding information
In-reply-to: <20130118013334.GE3074@awork2.anarazel.de>
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Comments: In-reply-to Andres Freund <andres@2ndquadrant.com>
	message dated "Fri, 18 Jan 2013 02:33:34 +0100"
Date: Thu, 17 Jan 2013 21:48:01 -0500
Message-ID: <23579.1358477281@sss.pgh.pa.us>
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Andres Freund <andres@2ndquadrant.com> writes:
> I have no problem requiring C code to use the even data, be it via hooks
> or via C functions called from event triggers. The problem I have with
> putting in some hooks is that I doubt that you can find sensible spots
> with enough information to actually recreate the DDL for a remote system
> without doing most of the work for command triggers.

"most of the work"?  No, I don't think so.  Here's the thing that's
bothering me about that: if the use-case that everybody is worried about
is replication, then triggers of any sort are the Wrong Thing, IMO.

The difference between a replication hook and a trigger is that a
replication hook has no business changing any state of the local system,
whereas a trigger *has to be expected to change the state of the local
system* because if it has no side-effects you might as well not bother
firing it.  And the fear of having to cope with arbitrary side-effects
occuring in the midst of DDL is about 80% of my angst with this whole
concept.

If we're only interested in replication, let's put in some hooks whose
contract does not allow for side-effects on the local catalogs, and be
done.  Otherwise we'll be putting in man-years of unnecessary (or at
least unnecessary for this use-case) work.

			regards, tom lane


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