Re: Strang behaviour SELECT ... LIMIT n FOR UPDATE

> -----Message d'origine-----
> De : Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
> Envoyé : mardi, novembre 27, 2007 23:46
> À : Daniel Caune
> Cc : pgsql-sql(at)postgresql(dot)org
> Objet : Re: [SQL] Strang behaviour SELECT ... LIMIT n FOR UPDATE
>
> "Daniel Caune" <daniel(dot)caune(at)ubisoft(dot)com> writes:
> > I'm facing a strange behaviour with a statement SELECT ... LIMIT n FOR
> > UPDATE in PostgreSQL 8.1. The number of rows returned is actually (n -
> > 1). I'm trying to find whether this is an identified issue with
> > PostgreSQL 8.1 that might have been fixed in a later version such as
> > 8.2; I don't have any problem in moving to a later version if needed.
>
> There's no known issue specifically of that form (and a quick test of
> 8.1 doesn't reproduce any such behavior). However, it is known and
> documented that LIMIT and FOR UPDATE behave rather oddly together:
> the LIMIT is applied first, which means that if FOR UPDATE rejects
> any rows as being no longer up-to-date, you get fewer than the expected
> number of rows out. You did not mention any concurrent activity in
> your example, but I'm betting there was some ...
>
> regards, tom lane

Yes, you were betting right. However I would have thought that the SELECT ... FOR UPDATE statement blocks if another process were locking the same rows.

The record values don't change from a call to another. I did read the documentation, especially the section that Bruce Momjian's pointed me out, but I don't think that it corresponds to this case (cf. my test).

I did the following test, removing all the where-clause from the SELECT statement. Every statement completes immediately, i.e. it doesn't block.

agoratokens=> select id from "Tokens"
id
-----
47
104
44
42
33
69
94
89
90
...

Time: 119.314 ms

agoratokens=> select id from "Tokens" limit 2 for update;
id
-----
47
104
(2 rows)

Time: 17.679 ms

agoratokens=> select id from "Tokens" limit 3 for update;
id
-----
47
104
(2 rows)

Time: 20.452 ms

The statement doesn't return the row where id equals to 44.

agoratokens=> select id from "Tokens" limit 3;
id
-----
47
104
44
(3 rows)

Time: 1.186 ms

The statement returns the row where id equals to 44.

agoratokens=> select id from "Tokens" limit 3 for update;
id
-----
47
104
(2 rows)

Time: 9.473 ms

The statement still doesn't return the row where id equals to 44.

agoratokens=> select id from "Tokens" where id IN (47, 104, 44, 42) limit 3 for update;
id
-----
47
104
44
(3 rows)

This time, the statement returns the row where id equals to 44.

agoratokens=> select id from "Tokens" limit 3;
id
-----
47
104
44
(3 rows)

Time: 7.547 ms

agoratokens=> select id from "Tokens" limit 5 for update;
id
-----
47
104
33
(3 rows)

Time: 11.725 ms

This time, the statement doesn't return the rows where id equals to 44 and 42.

agoratokens=> select id from "Tokens" limit 8 for update;
id
-----
47
104
33
69
94
89
(6 rows)

Time: 11.794 ms

The statement still doesn't return the rows where id equals to 44 a 42.

agoratokens=> select id from "Tokens" where id = 44 limit 3 for update;
id
----
44
(1 row)

Time: 14.172 ms

The statement does return the row where id equals to 44.

"However, it is known and documented that LIMIT and FOR UPDATE behave rather oddly together: the LIMIT is applied first, which means that if FOR UPDATE rejects any rows as being no longer up-to-date, you get fewer than the expected number of rows out."

Tom, when you say "rows as being no longer up-to-date", do you mean which values don't match anymore the where-clauses of the SELECT statement? If so, that doesn't correspond to my test since I remove every where-clause.

Any ideas, any other tests I can try?

Thanks,

--
Daniel