Date: Sat, 9 Nov 2019 14:10:13 +0200
From: Eugen Konkov <kes-kes@yandex.ru>
Message-ID: <322267502.20191109141013@yandex.ru>
To: Bruce Momjian <bruce@momjian.us>
CC: PostgreSQL-development <pgsql-hackers@postgreSQL.org>,
 pgsql-docs@lists.postgresql.org
Subject: Re: Does 'instead of delete' trigger support modification of OLD
In-Reply-To: <247609768.20191109140502@yandex.ru>
References: <919823407.20191029175436@yandex.ru>
 <20191106185935.GE1843@momjian.us>  
  <1771593631.20191107112032@yandex.ru> <352093181.20191107112429@yandex.ru>
  <20191107212655.GA487@momjian.us> <20191107222818.GA18712@momjian.us>
  <247609768.20191109140502@yandex.ru>
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Hello Eugen,

Saturday, November 9, 2019, 2:05:02 PM, you wrote:

> Hello Bruce,

> Friday, November 8, 2019, 12:28:18 AM, you wrote:

>> On Thu, Nov  7, 2019 at 04:26:55PM -0500, Bruce Momjian wrote:
>>> On Thu, Nov  7, 2019 at 11:24:29AM +0200, Eugen Konkov wrote:
>>> > >> As far as allowing DELETE to modify the trigger row for RETURNING, I am
>>> > >> not sure how much work it would take to allow that, but it seems like it
>>> > >> is a valid requite, and if so, I can add it to the TODO list.
>>> > 
>>> > > Yes,  Add please into TODO the feature to "allowing DELETE to modify the trigger row
>>> > > for  RETURNING".  Becuase, as I have described at first letter, without
>>> > > this the RETURNING rows **does not correspond actually deleted data**
>>> > 
>>> > > Thank you.
>>> 
>>> I have added a TODO item:
>>> 
>>>       Allow DELETE triggers to modify rows, for use by RETURNING 

>> Thinking some more on this, I now don't think a TODO makes sense, so I
>> have removed it.

>> Triggers are designed to check and modify input data, and since DELETE
>> has no input data, it makes no sense.  In the attached SQL script, you
>> can see that only the BEFORE INSERT trigger fires, so there is no way
>> even with INSERT to change what is passed after the write to RETURNING.
>> What you can do is to modify the returning expression, which is what I
>> have done for the last query --- hopefully that will help you.

> You  lost  my  idea.  First  of  all  I am talking about views and an
> INSTEAD OF triggers.

> INSERT/UPDATE operation present which data is added into DB
> DELETE operation present which data is deleted from DB
> (in  my  case I am not deleted exact that data which matched by where.
> See example below)

> Thus INSTEAD OF INSERT/UPDATE triggers are designed to check and modify input data
> eg.  we can insert/update something different then incoming data (here
> we are modifying NEW)

> Thus INSTEAD OF DELETE triggers are designed to check and delete **output** data
> eg. we can delete something different then underlaid data (here we are
> modifying OLD)

> for example, we have next data: 1 2 3 4 5 6 7 8
> it  is not presented by eight rows, but instead it is presented as one
> row with range data type: [1..8]

> When we insert data we will not get new row, we change current:
> insert into table values ( 9 ) will result
> [1..9]
> instead of
> [1..8]
> 9

> So  lets  look  into INSTEAD OF DELETE trigger when we deleting
> data:
> delete from table where x in ( 5, 6, 7 );
> after deleting this we should get:
> [1..4]
> [8..9]

> thus
> with t1 as ( delete from table where x in ( 5, 6, 7 ) returning * )
> select * from t1
> should return:
> [5..7]
> instead of
> [1..9]
> because we does not delete ALL [1..9], we just delete ONLY [5..7]

> Thus I need to change matched row OLD.x from [1..9] to [5..7]


> Please  reread  my  first  letter. There I describe more real life example
> when I am manipulating bi-temporal data.

> where some value exist at given period:
> id | app_period | value
> 7     [2019-01-01, 2019-04-05)    207

> And I am deleting third month: [ 2019-03-01, 2019-04-01 )
> with  t1  as  (  delete  from  table where app_period && [ 2019-03-01,
> 2019-04-01 ) returning * )
> select * from t1;
> 7   [ 2019-03-01, 2019-04-01 )   207

> select * from table;
> 7   [ 2019-01-01, 2019-03-01 )   207
> 7   [ 2019-04-01, 2019-04-05 )   207

Here when data is deleted the next row is matched:
   7     [2019-01-01, 2019-04-05)    207
and assigned to OLD.
Because I am deleting data ONLY from [ 2019-03-01, 2019-04-01 ) period
I am required to change OLD:

OLD.app_period = [ 2019-03-01, 2019-04-01 )

So I should get:
> 7   [ 2019-03-01, 2019-04-01 )   207
instead of
> 7     [2019-01-01, 2019-04-05)    207


-- 
Best regards,
Eugen Konkov