Subject: Rules and Command Status - update/insert/delete rule with series of commands in action

From: "[email-masked]" <[email-masked]>
Date: Thu, 30 May 2024 14:32:46 -0400

The RULE infrastructure permits the programmer to specify a series of 
commands in the DO action

from the syntax diagram in the manual :

CREATE [ OR REPLACE ] RULE name AS ON event
     TO table [ WHERE condition ]
     DO [ ALSO | INSTEAD ] { NOTHING | command | ( command ; command ... ) }

The manual described the series of commands as the "rule action" ,  
implying (to me) that there is a sense in which the entire series 
comprising the action is one operation.

I am specifically interested in the case of update and an example of an 
unconditional rule such as

CREATE or REPLACE RULE multi-action AS ON UPDATE TO my_view
     DO INSTEAD (
     UPDATE my_table_a
         SET a_column = value
        WHERE OLD.keycolumn = keyvalue;
     UPDATE my_table_b
         SET b_column = value
        WHERE OLD.keycolumn = keyvalue;
   );

where my intention is that one and only one of the action commands 
should update any row.

This all works except for one thing :   the final status, including the 
(for me) all-important number of rows updated.

It turns out that in this example,     if the UPDATE my_table_b updates 
one (or more) rows,    the status shows that number,   but if the UPDATE 
my_table_b updates no rows,    the status shows 0 rows updated,   even 
if one (or more) rows of my_table_a were successfully updated by the 
first command.     This is not what I want.

The chapter entitled "Rules and Command Status "  (approximately chap 
number 41.6 depending on version) says

"If there is any unconditional|INSTEAD|rule for the query, then the 
original query will not be executed at all. In this case, the server 
will return the command status for the last *query* that was inserted by 
an|INSTEAD|rule (conditional or unconditional) and is of the same 
command type (|INSERT|,|UPDATE|, or|DELETE|) as the original query."     
(my bold of the word query).

But what is a query in this context?        In my example,    is the 
last *query* the

. *action* of the last unconditional RULE which executed  (only one in 
my example but there could be other applicable rules for update of my_view)

OR

.   last *command* of the series of commands comprising theaction of the 
last unconditional RULE which executed

?

Well, I assume what postgresql actually does is the latter, but surely 
there is a case for it to be the former, where the rows_updated of the 
action would be the sum of all rows updated by all commands in that 
action's series.  .   In my example,    postgresql is telling the 
application that no rows were updated when actually one (or more) row 
was updated, and the sum of all rows updated is one.

Any thoughts?      Any rationales one way or the other?    Any interest 
in perhaps providing a choice via a configuration parameter?

Cheers, John Lumby

---

From: "David G. Johnston" <[email-masked]>
Date: Thu, 30 May 2024 14:56:06 -0600

On Thu, May 30, 2024, 12:32 [email-masked] <[email-masked]>
wrote:

> Any thoughts?
Very little interest exists in working on user-specified rules.  They are
practically deprecated.

>
>     Any interest in perhaps providing a choice via a configuration
> parameter?
>

Almost certainly not.  Configuration should not affect query behavior.  If
anything is done it would have to be new syntax.  Though I haven't explored
the use case presented.  See the first point.  I sure don't have the
requisite familiarity here.

David J.