Re: BUG #17827: Rule on insert into table doesn't work when excepting select from the table itself

Hello Tom,
Thank you for the answer. Now it is more clear for me.

According to this information I can cause another kind of error:

--drop table tbl cascade;

--drop table rule_stat cascade;

create table tbl (id serial primary key, msg text);

create table rule_stat (msg text, id int references tbl(id));

create rule rule_tbl as on insert to tbl do insert into rule_stat values('Last
inserted id was ',new.id);

insert into tbl (msg)

select 'I`m an insert';

Result:

SQL Error [23503]: ERROR: insert or update on table "rule_stat" violates
foreign key constraint "rule_stat_id_fkey"
Detail: Key (id)=(2) is not present in table "tbl".

Such behaviour is a bit confusing. Because by using new.* I expect to get a
recently inserted row, but not the result of some query running the second
time (which causes implicit incrementing of id serial).

Best regards,

Hanna

On Thu, Mar 9, 2023 at 5:01 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> > create table tbl (id int);
> > create table rule_stat (msg text, id int);
> > create rule rule_tbl as on insert to tbl do insert into rule_stat
> > values('Rule triggered for ',new.id);
>
> > insert into tbl
> > select 1
> > except
> > select id from tbl;
>
> > table rule_stat; -- no rows
>
> This is not a bug. The DO ALSO command executes after the original
> INSERT command, and what it executes looks basically like
>
> insert into rule_stat
> select 'Rule triggered for ', id from
> (select 1
> except
> select id from tbl);
>
> But at this point we've already completed the original INSERT,
> so now there is a row with id 1 in "tbl", and thus the EXCEPT
> produces nothing.
>
> While there are use-cases for this sort of behavior, most people
> find that propagating data to another table is better done with
> an AFTER trigger. It's far easier to understand what will happen.
>
> regards, tom lane
>

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