Обсуждение: BUG #17085: Should be able to create an index without referential checking when ON DELETE NO ACTION
BUG #17085: Should be able to create an index without referential checking when ON DELETE NO ACTION
От
PG Bug reporting form
Дата:
The following bug has been logged on the website: Bug reference: 17085 Logged by: Victor Porton Email address: porton@narod.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. Therefore in this case ALTER TABLE should be able to create an index without 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.
Re: BUG #17085: Should be able to create an index without referential checking when ON DELETE NO ACTION
От
"David G. Johnston"
Дата:
On Tue, Jul 6, 2021 at 3:29 PM PG Bug reporting form <noreply@postgresql.org> wrote:
The following bug has been logged on the website:
Bug reference: 17085
Logged by: Victor Porton
Email address: porton@narod.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.
Re: BUG #17085: Should be able to create an index without referential checking when ON DELETE NO ACTION
От
Pantelis Theodosiou
Дата:
On Wed, Jul 7, 2021 at 12:32 AM David G. Johnston <david.g.johnston@gmail.com> wrote:
On Tue, Jul 6, 2021 at 3:29 PM PG Bug reporting form <noreply@postgresql.org> wrote:The following bug has been logged on the website:
Bug reference: 17085
Logged by: Victor Porton
Email address: porton@narod.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 indexwithout 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
Re: BUG #17085: Should be able to create an index without referential checking when ON DELETE NO ACTION
От
"David G. Johnston"
Дата:
On Tue, Jul 6, 2021 at 5:05 PM Pantelis Theodosiou <ypercube@gmail.com> wrote:
Transactions is referenced twice, once without correlation and secondly with correlation.
Yeah, the USING clause of DELETE is basically an INNER JOIN. If you need an ANTI-JOIN you can write that via NOT EXISTS just like you would in a normal SELECT query. You don't get the option to write an anti-join using a left join and testing for null.
David J.
On 7/7/21 2:32 AM, David G. Johnston wrote:
That's a typo, I meant foreign keys, not indexes.On Tue, Jul 6, 2021 at 3:29 PM PG Bug reporting form <noreply@postgresql.org> wrote:The following bug has been logged on the website:
Bug reference: 17085
Logged by: Victor Porton
Email address: porton@narod.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 indexwithout 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.
Anyway, after following my advice PostgreSQL would become more "logical" and more convenient for users.
There is no reason to have different rules for ALTER TABLE and for DELETE/UPDATE.
Re: BUG #17085: Should be able to create an index without referential checking when ON DELETE NO ACTION
От
"David G. Johnston"
Дата:
On Tue, Jul 6, 2021 at 5:33 PM Victor Porton <porton.victor@gmail.com> wrote:
That's a typo, I meant foreign keys, not indexes.
OK, then add the "NOT VALID" qualifier to your ALTER TABLE command. But you will have to perform validation at some point...
Anyway, after following my advice PostgreSQL would become more "logical" and more convenient for users.
There is no reason to have different rules for ALTER TABLE and for DELETE/UPDATE.
It's hard to follow the advice contained in an email that doesn't seem to understand how things work today.
The subject of the report - which isn't a bug by the way - is to skip referential checking when adding an index. PostgreSQL doesn't do that today...
David J.