Re: Is the Halloween problem an issue in Postgres

On Wed, Dec 02, 2020 at 02:08:41PM -0800, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com> wrote:

> On 12/2/20 2:02 PM, Thomas Kellerer wrote:
> > guyren(at)icloud(dot)com schrieb am 02.12.2020 um 21:27:
> > > The Halloween problem is that it is a challenge for the database if
> > > you’re updating a field that is also in the WHERE clause of the same
> > > query.
> > >
> > > I just saw a presentation from someone about how in SQL Server he
> > > recommended writing changes to a temp table and then writing them to
> > > the table as being much more efficient.
> >
> > It sounds strange to me, that this _is_ actually a problem.
> >
> > Why exactly is that a problem in SQL Server?
>
> Yeah that was a new one to me. A quick search found:
>
> https://www.sqlshack.com/the-halloween-problem-in-sql-server-and-suggested-solutions/
>
> > And what are the consequences if you do it nevertheless.

It looks like the anser is no (unless I've misunderstood the problem):

create table a (id serial not null primary key, a integer not null, b integer not null);
create index a_a on a(a);
insert into a (a, b) values (1, 2);
insert into a (a, b) values (2, 3);
insert into a (a, b) values (3, 4);
insert into a (a, b) values (4, 5);
insert into a (a, b) values (5, 6);
insert into a (a, b) values (6, 7);
update a set a = a + 1 where a < 4;
select * from a order by id;
drop table a cascade;

results in:

id | a | b
----+---+---
1 | 2 | 2
2 | 3 | 3
3 | 4 | 4
4 | 4 | 5
5 | 5 | 6
6 | 6 | 7

It's the same with or without the index on a(a).

cheers,
raf