On 12/2/20 4:23 PM, raf wrote:
On Wed, Dec 02, 2020 at 02:08:41PM -0800, Adrian Klaver <adrian.klaver@xxxxxxxxxxx> wrote:
On 12/2/20 2:02 PM, Thomas Kellerer wrote:
guyren@xxxxxxxxxx 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).
The Halloween Problem does not seem to cause the statement to fail, but to
run slowly.
--
Angular momentum makes the world go 'round.
