Re: Insert works but fails for merge

So it looks like the OP does not mind updating more than one row. If you
want to keep it simple and not do a lot of casting, consider using a CTE to
do a reverse-upsert and use a prepared statement. Prepare and cast once,
and have your app send the raw uncasted strings many, many times:

prepare foo(text,int,timestamptz) as with x as (update tab1 set mid=$2
where id=$1 returning 1)
insert into tab1 select $1,$2,$3 where not exists (select 1 from x);

execute foo('5efd4c91-ef93-4477-840c-a723ae212d99', 123,
'2024-08-09T11:33:49.402585600Z');
execute foo('some_other_id', 456, '2024-08-11T21:44:55.8675309Z');
etc.

Your app/driver may or may not already do protocol-level statement
prepare/execute automagically, so test that way first.

It's version 15.4 postgres.

Keep on the latest revision. Right now, that's 15.8. Upgrading revisions is
quick and painless.

Cheers,
Greg