Re: MERGE behavior with REPEATABLE READ isolation level

On Sun, Feb 15, 2026 at 3:20 AM Alexander Korotkov <aekorotkov@gmail.com>
wrote:

> Hi hackers,
>
> I found it strange that ExecMergeMatched() checks for
> IsolationUsesXactSnapshot() in the TM_Deleted case, but not in the
> TM_Updated case.  Indeed, EPQ works on the repeatable read isolation level!
>
> s1# create table test (id int primary key, val int);
> s1# insert into test values (1,0);
>
> s2# begin;
> s2# update test set val = val + 100;
>
> s1# MERGE INTO test t USING (VALUES (1, 100)) AS s (id, inc)
>       ON t.id = s.id
>     WHEN MATCHED THEN
>       UPDATE SET val = t.val + s.inc
>     WHEN NOT MATCHED THEN
>       INSERT (id, val) VALUES (s.id, s.inc);
> (waiting ...)
>
> s2# commit;
> s1# MERGE 1
> s1# select * from test;
>  id | val
> ----+-----
>   1 | 200
> (1 row)
>

Oh, sorry I missed the begin statement for s1.  The complete case should
look like this.

s1# create table test (id int primary key, val int);
s1# insert into test values (1,0);

  s2# begin;
  s2# update test set val = val + 100;

s1# begin isolation level repeatable read;
s1# MERGE INTO test t USING (VALUES (1, 100)) AS s (id, inc)
      ON t.id = s.id
    WHEN MATCHED THEN
      UPDATE SET val = t.val + s.inc
    WHEN NOT MATCHED THEN
      INSERT (id, val) VALUES (s.id, s.inc);
(waiting ...)

  s2# commit;

s1# MERGE 1
s1# select * from test;
 id | val
----+-----
  1 | 200
(1 row)

------
Regards,
Alexander Korotkov
Supabase