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

PG Bug reporting form <noreply@postgresql.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

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@sss.pgh.pa.us> wrote:

PG Bug reporting form <noreply@postgresql.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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Hanna Yanchurevich <hyanchurevich@spotware.com> writes:

According to this information I can cause another kind of error:
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';
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".

Yup. It's pretty obvious what's going on if you look at EXPLAIN:

explain verbose insert into tbl (msg)
select 'I`m an insert';
QUERY PLAN
------------------------------------------------------------------------
Insert on public.tbl (cost=0.00..0.01 rows=0 width=0)
-> Result (cost=0.00..0.01 rows=1 width=36)
Output: nextval('tbl_id_seq'::regclass), 'I`m an insert'::text

Insert on public.rule_stat (cost=0.00..0.01 rows=0 width=0)
-> Result (cost=0.00..0.01 rows=1 width=36)
Output: 'Last
inserted id was '::text, nextval('tbl_id_seq'::regclass)
(8 rows)

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).

If that's the mental model you want to work with, use a trigger.
When you work with rules, you are working with macros, and they
have the same sort of multiple-evaluation hazards as macros in,
say, C.

regards, tom lane