Re: Confirmation on concurrent SELECT FOR UPDATE with ON CONFLICT DO NOTHING

On 4/29/26 4:07 PM, Matt Magoffin wrote:
> Hello, I was hoping to confirm some transaction behaviour I am seeing (in Postgres 17) in read-committed isolation mode that caught me off guard is, in fact, expected. First some setup:
>
> CREATE TABLE txtest (id INTEGER NOT NULL PRIMARY KEY);
> INSERT INTO txtest (id) VALUES (1);
>
> Then in one session, I run:
>
> BEGIN; SELECT * FROM txtest WHERE id = 1 FOR UPDATE;
>
> Then, in a different session, I run:
>
> INSERT INTO txtest
> SELECT id
> FROM (VALUES
> (1),
> (2)
> ) AS t(id)
> ON CONFLICT
> DO NOTHING;
>
> This completes immediately, with
>
> INSERT 0 1
>
> and indeed there are 2 rows now in that session:
>
> SELECT * FROM txtest;
> id
> ----
> 1
> 2
>
> This is what caught be off guard, as I had been thinking the INSERT would block until the first session’s transaction finished. Now, back in session #1, I run:
>
> DELETE FROM txtest WHERE ID = 1; COMMIT;
>
> Now in both sessions there is 1 row, with “2”, where I had been hoping to end up with both “1” and “2” after the INSERT waited for the SELECT … FOR UPDATE to complete first.
>
> If I change session #1’s query from SELECT … FOR UPDATE to an immediate DELETE, I get what I expected, i.e.
>
> BEGIN; DELETE FROM txtest WHERE id = 1;
>
> Then in session #1 the same INSERT … ON CONFLICT DO NOTHING statement blocks until session #1 commits, and it results in
>
> INSERT 0 2
>
> The difference in transaction behaviour between SELECT … FOR UPDATE and DELETE I did not understand from the documentation, so would appreciate any confirmation/clarification/insight on what I’m seeing so I can better understand.

From here:
https://www.postgresql.org/docs/17/explicit-locking.html#LOCKING-ROWS

"FOR UPDATE

FOR UPDATE causes the rows retrieved by the SELECT statement to be
locked as though for update. This prevents them from being locked,
modified or deleted by other transactions until the current transaction
ends. That is, other transactions that attempt UPDATE, DELETE, SELECT
FOR UPDATE, SELECT FOR NO KEY UPDATE, SELECT FOR SHARE or SELECT FOR KEY
SHARE of these rows will be blocked until the current transaction ends;
conversely, ..."

Nothing about an INSERT.

And from here:

https://www.postgresql.org/docs/current/sql-insert.html#SQL-ON-CONFLICT

ON CONFLICT DO NOTHING simply avoids inserting a row as its alternative
action.

So in your first case the INSERT is never done and there is no lock for
the INSERT in any case.

>
> Thank you,
> Matt Magoffin
>
>
>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com