Re: Reject WAIT FOR earlier in transaction-snapshot mode

Hi Sami,

On Wed, Sep 9, 2026 at 3:55 AM Sami Imseih <samimseih(dot)pg(at)gmail(dot)com> wrote:
>
> Hi,
>
> The WAIT FOR docs state:
>
> "WAIT FOR must be executed as a top-level command. It cannot be
> executed from a function, procedure, or DO block. It also requires
> that no active or registered snapshot be held, and therefore cannot be
> used in contexts where such a snapshot must remain active, including
> transactions running at isolation levels higher than READ COMMITTED."
>
> So this reads as any transaction above READ COMMITTED should reject WAIT FOR.
> However, in REPEATABLE READ and SERIALIZABLE, you are allowed to call
> WAIT FOR before taking the snapshot.
>
> ```
> postgres=# BEGIN ISOLATION LEVEL REPEATABLE READ;
> BEGIN
> postgres=*# WAIT FOR LSN '0/0' WITH (MODE 'primary_flush');
> status
> ---------
> success
> (1 row)
>
> postgres=*# select 1;
> ?column?
> ----------
> 1
> (1 row)
>
> postgres=*# WAIT FOR LSN '0/0' WITH (MODE 'primary_flush');
> ERROR: WAIT FOR must be called without an active or registered snapshot
> DETAIL: WAIT FOR cannot be executed within a transaction with an
> isolation level higher than READ COMMITTED.
> postgres=!#
> ```
>
> That does not match the documented behavior, and I do not think there is
> a reason to allow WAIT FOR in such a transaction before the snapshot is
> taken.

Thanks for the patch! The rationale LGTM.

I've two comments for it:

1) Separate the isolation check from the snapshot check
I am wondering whether it could be helpful to move the isolation level
check to the place after the top level statement checking. It has two
pros -- one is to distinguish isolation levels higher than READ
COMMITTED from READ COMMITTED with snapshots remaining unpopped;
another is to reject early in order to save some parsing effort for
unsupported use.

if (IsolationUsesXactSnapshot())
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("WAIT FOR cannot be executed in a transaction with
an isolation level higher than READ COMMITTED")));

2) Valid the wait mode in test

+$node_standby->psql(
+ 'postgres',
+ qq[
+BEGIN ISOLATION LEVEL REPEATABLE READ;
+WAIT FOR LSN '${lsn3}' WITH (MODE 'primary_flush');
+],

Primary flush is not a valid mode in standby, so it will still raise
errors without the patch, though the error message is different. After
the patch, the stability of this test relies on the order of errors
between mode and isolation level checking. It might be helpful to
change the mode to default(replay) like WAIT FOR LSN '${lsn3}';

--
Regards,
Xuneng Zhou
HighGo Software Co., Ltd.