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Reverse foreign key cascading (or how to collect database garbage)


Reverse foreign key cascading (or how to collect database garbage)

By : Yoisbel Hurtado
Date : November 19 2020, 03:01 PM
I hope this helps . There's no need for mark and sweep batch processing since the following query will do the job as long as BLAHS.PK can never contain a comma:
code :
delete from blahs 
where not exists (select 1 from foo where ','||some_blahs||',' like '%,'||blahs.pk||',%')
  and not exists (select 1 from bar where bar.blah = blahs.pk)
  and not exists (select 1 from baz where blahs.pk in (baz.a_blah, baz.another_blah));
create table foo_blahs ( foo_pk varchar2(20)  references foo on delete cascade
                       , blah_pk varchar2(20) references blahs
                       , constraint foo_blah_pk primary key (foo_pk, blah_pk) enable);
insert into foo (pk, some_blahs) values ('a_foo', 'a,b,c');
insert into foo (pk) values ('a_foo');
insert into foo_blahs (foo_pk, blah_pk) values ('a_foo', 'a');
insert into foo_blahs (foo_pk, blah_pk) values ('a_foo', 'b');
insert into foo_blahs (foo_pk, blah_pk) values ('a_foo', 'c');
delete from blahs 
where not exists (select 1 from foo_blahs where foo_blahs.blah_pk = blahs.pk)
  and not exists (select 1 from bar where bar.blah = blahs.pk)
  and not exists (select 1 from baz where blahs.pk in (baz.a_blah, baz.another_blah));


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Why doesn't the Java garbage collector collect garbage?

Why doesn't the Java garbage collector collect garbage?


By : Ayoub Mhaidir
Date : March 29 2020, 07:55 AM
I wish did fix the issue. Your question is somewhat unclear - are you actually getting OutOfMemoryErrors? What are you trying to solve? There are a few reasons you are seeing the behavior manifested in your test case:
Foreign Keys cascading in JPA or database Schema

Foreign Keys cascading in JPA or database Schema


By : Renzo Alpuerto
Date : March 29 2020, 07:55 AM
may help you . You should always have a 2 level check. if you do not use the features of JPA, then it's a big waste of the functionality JPA provides. you should actually make sure that you JPA relations match your DB relations as closely as possible. It will help you a lot as JPA can cache data and even prevent unnecessary calls to DB. eg if u have a not null constraint and you persist with no JPA constraint, your DB has to do all the work and throw the exception back. normally in an application, the network and DB are the slowest factors in the app. so you should try mimicking the constraints in JPA to avoid unnecessary overhead. also using such constraints you can form bidirectional relationships and have collections of associated entities and many more such advantages.
Who will garbage collect or unload the garbage collector

Who will garbage collect or unload the garbage collector


By : Jonathan Nelson
Date : March 29 2020, 07:55 AM
around this issue When you start any process, the memory for that is coming from the operating system.
When that process ends, all of the memory goes back to that operating system.
Fetching reverse foreign key of reverse foreign key in django while still taking advantage of prefetch_related

Fetching reverse foreign key of reverse foreign key in django while still taking advantage of prefetch_related


By : Theo
Date : March 29 2020, 07:55 AM
NHibernate - does it work well with database-level cascading deletions on foreign key constraints

NHibernate - does it work well with database-level cascading deletions on foreign key constraints


By : SephGrave
Date : March 29 2020, 07:55 AM
This might help you NHibernate simply treats your RDBMS as a data store. In most cases, this means that you can let NHibernate do the work for you.
Let NHibernate do the cascading deletes. That way you can keep your logic and control where it belongs (in your application), rather than in the database where your application has little control or flexibility.
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