Re: Is the Halloween problem an issue in Postgres

On 12/2/20 4:23 PM, raf wrote:
> On Wed, Dec 02, 2020 at 02:08:41PM -0800, Adrian Klaver <adrian.klaver@aklaver.com> wrote:
>
>> On 12/2/20 2:02 PM, Thomas Kellerer wrote:
>>> guyren@icloud.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).

The Halloween Problem does not seem to cause the statement to fail, but to 
run slowly.

-- 
Angular momentum makes the world go 'round.