BUG #19359: Row level security: Upserts require insert policies in the update path

The following bug has been logged on the website:

Bug reference: 19359
Logged by: Elena Krippner
Email address: elena(at)cedardb(dot)com
PostgreSQL version: 18.1
Operating system: Ubuntu 25.10
Description:

The documentation on policies
(https://www.postgresql.org/docs/current/sql-createpolicy.html) says for
upserts:
Note that INSERT with ON CONFLICT DO UPDATE checks INSERT policies' WITH
CHECK expressions only for rows appended to the relation by the INSERT path.

In this case, the update path is taken, but the values can only be upserted
after adding an insert policy:
root(at)4284b66b43be:/# psql -h localhost -U postgres
psql (18.1 (Debian 18.1-1.pgdg13+2))
Type "help" for help.

postgres=# create table t (a integer primary key);
CREATE TABLE
postgres=# insert into t values (1), (2), (3);
INSERT 0 3
postgres=# create role policy_role;
CREATE ROLE
postgres=# grant select, insert, update, delete on t to policy_role;
GRANT
postgres=# alter table t enable row level security;
ALTER TABLE
postgres=# create policy t_aLess4 on t for update using (true) with check (a
< 4);
CREATE POLICY
postgres=# create policy t_select on t for select using (true);
CREATE POLICY
postgres=# set role policy_role;
SET
postgres=> select * from t order by a;
a
---
1
2
3
(3 rows)

postgres=> insert into t values (1) on conflict (a) do update set a = 0;
ERROR: new row violates row-level security policy for table "t"
postgres=> set role postgres;
SET
postgres=# create policy t_insert on t for insert with check (true);
CREATE POLICY
postgres=# set role policy_role;
SET
postgres=> insert into t values (1) on conflict (a) do update set a = 0;
INSERT 0 1
postgres=> select * from t order by a;
a
---
0
2
3
(3 rows)