Re: Trigger.sgml

On 1/28/16 8:02 PM, Tatsuo Ishii wrote:
> I am working as a volunteer to translate docs to Japanese. I have been
> having hard time to parse the following sentence in
> doc/src/sgml/trigger.sgml.
>
> --------------------------------------------------------------------
> The possibility of surprising outcomes should be considered when there
> are both <literal>BEFORE</> <command>INSERT</command> and
> <literal>BEFORE</> <command>UPDATE</command> row-level triggers that
> both affect a row being inserted/updated (this can still be
> problematic if the modifications are more or less equivalent if
> they're not also idempotent).
> --------------------------------------------------------------------
>
> Especially I don't understand this part:
>
>    (this can still be problematic if the modifications are more or less
>    equivalent if they're not also idempotent).
>
> It would be great if someone could enligntend me.

I believe the idea here is that thanks to UPSERT you can now get very 
strange behavior if you have BEFORE triggers that aren't idempotent. IE:

CREATE TABLE test(  a int PRIMARY KEY
);

BEFORE INSERT a = a - 1
BEFORE UPDATE a = a + 1

INSERT (1) -- Results in 0
INSERT (2) -- Results in 1

Now if you try to UPSERT (1), the before insert will give you a=0, which 
conflicts. So then you end up with an UPDATE, which gives you a=1 again. 
Things are even worse when you try to UPSERT (2), because the insert 
conflicts but then you try to update a row that doesn't exist (a=2).

Obviously this is a ridiculous example, but hopefully it shows the problem.
-- 
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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