Re: BUG #17085: Should be able to create an index without referential checking when ON DELETE NO ACTION

On Wed, Jul 7, 2021 at 12:32 AM David G. Johnston <
david(dot)g(dot)johnston(at)gmail(dot)com> wrote:

> On Tue, Jul 6, 2021 at 3:29 PM PG Bug reporting form <
> noreply(at)postgresql(dot)org> wrote:
>
>> The following bug has been logged on the website:
>>
>> Bug reference: 17085
>> Logged by: Victor Porton
>> Email address: porton(at)narod(dot)ru
>> PostgreSQL version: 13.3
>> Operating system: Linux
>> Description:
>>
>> When there is either ON DELETE NO ACTION or ON UPDATE NO ACTION for a
>> foreign key, referential integrity is not preserved anyway.
>>
>
> Yes, it is preserved, the delete or update is prevented.
>
>
>> Therefore in this case ALTER TABLE should be able to create an index
>
> without referential checking.
>>
>
> Indexes are single table only and thus do not care about referential
> integrity or perform any referential checking.
>
>>
>> It is especially useful for removing superfluous data like:
>>
>> delete from tags using transactions where not exists(select * from
>> transactions where tags.tx_id=transactions.id);
>>
>> I've got into a trouble: This command runs too long because of no foreign
>> key, but I can't create a foreign key because this command didn't run yet
>> to
>> make referntial integrity working.
>>
>>
> Foreign keys likewise don't affect performance...they are strictly
> concerned with data updates. Now, a FK column is not indexed by default so
> depending on how queries are written, and the data in for the FK, there may
> be a performance gain from adding an index.
>
> All that is to say if you want to turn a non-FK column into an FK you may
> find it helpful to add the index first, fix the data, then add the FK.
>
> David J.
>

Besides all the good advice by David J., your delete statement has some
issues because it's written more complex than it could.
Transactions is referenced twice, once without correlation and secondly
with correlation.
As a result, if transactions has 0 rows, the statement would delete no rows
at all from tags (although I suppose you'd want to delete them all).
And if transactions is big enough, you may not get an efficient plan.

I suggest you try using this rewrite (besides adding the suggested index):

delete from tags
where not exists (select * from
transactions where tags.tx_id=transactions.id);

Pantelis