Re: NOT DEFERRABLE constraints are checked before command finishes

UNIQUE constraints have this behaviour. It is explained in the section:

Non-Deferred Uniqueness Constraints

When a UNIQUE or PRIMARY KEY constraint is not deferrable, PostgreSQL checks
for uniqueness immediately whenever a row is inserted or modified. The SQL
standard says that uniqueness should be enforced only at the end of the
statement; this makes a difference when, for example, a single command
updates multiple key values. To obtain standard-compliant behavior, declare
the constraint as DEFERRABLE but not deferred (i.e., INITIALLY IMMEDIATE).
Be aware that this can be significantly slower than immediate uniqueness
checking.

On Wed, Jul 14, 2021 at 9:29 AM PG Doc comments form <noreply(at)postgresql(dot)org>
wrote:

> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/13/sql-createtable.html
> Description:
>
> According to the docs:
>
> A constraint that is not deferrable will be checked immediately after every
> command.
>
> But this is the behavior I observe on PG 13.3:
>
> create table t (n int primary key);
> insert into t values (1), (2), (3);
> update t set n = n + 1;
> ERROR: 23505: duplicate key value violates unique constraint "t_pkey"
> DETAIL: Key (n)=(2) already exists.
>
> If the constraint was checked *after* the command it should work. It
> appears
> it is checked before the command has finished.
>
> In contrast a DEFERRABLE INITIALLY IMMEDIATE constraint which is documented
> as "If the constraint is INITIALLY IMMEDIATE, it is checked after each
> statement." behaves as expected.
>
> create table t (n int primary key deferrable initially immediate);
> insert into t values (1), (2), (3);
> update t set n = n + 1; --> UPDATE 3
>