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Re: Why can't I put a BEFORE EACH ROW trigger on a view?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Karl O(dot) Pinc" <kop(at)meme(dot)com>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: Why can't I put a BEFORE EACH ROW trigger on a view?
Date: 2007-02-25 01:48:58
Message-ID: 960.1172368138@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-general
"Karl O. Pinc" <kop(at)meme(dot)com> writes:
> why can't I put a SELECT rule on a table?

Because then it would be a view.

As for $SUBJECT, the problem is that there will never be an insert into
a view --- not at the level of a physical insert attempt anyway --- and
thus there is nothing for a trigger to do.

The reason there will never be an insertion trigger event is that we
reject any INSERT on a view that isn't rewritten (by an unconditional
DO INSTEAD rule) into something else.

I recall a prior discussion about making it possible to use triggers on
views as a substitute for DO INSTEAD rules, by removing the rewrite-time
check and only erroring out if we actually get to the point of
attempting a physical insert.  Then a BEFORE INSERT trigger could do
something appropriate with the data and return NULL to prevent the
error.  This seems like a good idea because triggers often are much
easier to work with than rules --- eg, there's no problem with multiple
evaluations of volatile functions, even if you send the data to several
places. However, I'm not sure that the idea scales to cover updates and
deletes; with no concept of physical tuple identity (ctid) for the view
rows, it's not clear that you can write triggers that will reliably do
the right things.

			regards, tom lane

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Next:From: Joshua D. DrakeDate: 2007-02-25 01:56:34
Subject: Re: Why can't I put a BEFORE EACH ROW trigger on a view?
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