Обсуждение: SELECT FOR UDPATE behavior inside joins

"Khan, Tanzeel" <tzlkhan@amazon.com> writes:
> I am trying to understand the SELECT FOR UPDATE behavior when it is not returning rows back to client.

> postgres=> CREATE TABLE t (col1 INT, col2 INT);
> postgres=> INSERT INTO t VALUES (1, 1);

> S1: BEGIN; UPDATE t SET col2 = col2 + 1 WHERE col1 = 1;
> S2: BEGIN; WITH cte AS (SELECT * FROM t WHERE col1 = 1 FOR UPDATE) UPDATE t SET col2 = t.col2 + 1 FROM cte AS
t_self_joinWHERE (t.col2 = t_self_join.col2); 
> S1: COMMIT;
> S2: zero rows updated

> Why does session 2 update zero rows ?

Since the CTE has FOR UPDATE, it blocks and returns the updated-by-S1
version of the row.  But the outer query initially reads the old
version of the row, so the join condition fails, and we never get
to the lock-row-and-recheck behavior of UPDATE.

I am not sure what you are hoping to accomplish with that self-join.
I suppose this is an oversimplified example, but it's too
oversimplified for anyone to see why you'd want to do it like that.

            regards, tom lane

On 12/29/25 01:15, Khan, Tanzeel wrote:
> Hi,
> 
> I am trying to understand the SELECT FOR UPDATE behavior when it is not 
> returning rows back to client.
> 
> postgres=> CREATE TABLE t (col1 INT, col2 INT);
> postgres=> INSERT INTO t VALUES (1, 1);
> 
> S1: BEGIN; UPDATE t SET col2 = col2 + 1 WHERE col1 = 1;
> S2: BEGIN; WITH cte AS (SELECT * FROM t WHERE col1 = 1 FOR UPDATE) 
> UPDATE t SET col2 = t.col2 + 1 FROM cte AS t_self_join WHERE (t.col2 = 
> t_self_join.col2);
> S1: COMMIT;
> S2: zero rows updated
> 
> Why does session 2 update zero rows ? Shouldn’t the SELECT FOR UPDATE 
> and UPDATE read the new version of the row as per
>  > If so, the second updater proceeds with its operation using the 
> updated version of the row. In the case of SELECT FOR UPDATE and SELECT 
> FOR SHARE, this means it is the updated version of the row that is 
> locked and returned to the client.
> https://www.postgresql.org/docs/current/transaction-iso.html#XACT-READ- 
> COMMITTED <https://www.postgresql.org/docs/current/transaction- 
> iso.html#XACT-READ-COMMITTED>
> 
> Does this mean the new version for row is only returned when the SELECT 
> FOR SHARE is returning rows back to client ?

No I think  you missed the sentence before the one you you show above:

"The search condition of the command (the WHERE clause) is re-evaluated 
to see if the updated version of the row still matches the search 
condition. If so, the second updater proceeds with its operation using 
the updated version of the row."

So:

  WHERE (t.col2 = t_self_join.col2)

The S1 UPDATE changed the value of col2 to something different then what 
is is found by WITH cte AS (SELECT * FROM t WHERE col1 = 1 FOR UPDATE).

> 
> ------
> Thanks,
> Tanzeel


-- 
Adrian Klaver
adrian.klaver@aklaver.com