> 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.
Thank you for the info!
Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp