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
[email protected]
