Thread: finding if a foreign key is in use
in my app i have a table where the id serves as a foreign key for one or more other tables. if i want to delete a row in the table, i currently search the other tables where this table is referenced to see if the row is in use - and then, if not in use, permit deletion. Now if i want the delete button in my app to be disabled whenever a row that is in use is selected, searching the database every time would dramatically slow down the app. is there some sql query that will immediatly give an answer as to whether the row is in use or not? -- regards kg http://www.onlineindianhotels.net - hotel bookings reservations in over 4600 hotels in India http://www.ootygolfclub.org
Kenneth Gonsalves <lawgon@thenilgiris.com> wrote: > in my app i have a table where the id serves as a foreign key for > one or more other tables. if i want to delete a row in the table, > i currently search the other tables where this table is referenced > to see if the row is in use - and then, if not in use, permit > deletion. You can automate this check by declaring the foreign keys like this: id some_type references other_table(id) on delete no action The "on delete no action" bit means "if you try to delete the row in the referred-to table (where it is the primary key),cause an error. The alternative is "on delete cascade", which means that rows in the referring tables are deleted ifthe row that they refer to is deleted. > Now if i want the delete button in my app to be disabled whenever > a row that is in use is selected, searching the database every time > would dramatically slow down the app. Basically you do have to do this search. But it won't be too slow if you create an index on the foreign key. I would probably do it like this. Here is the first table: create table T1 ( id integer primary key, x text ); Here is the second table that refers to it: create table T2 ( id integer references T1.id on delete no action, y text ); So that the searches can be efficient, we create an index: create index T2_by_id on T2(id); Now I would create a view that adds an extra column to T1, indicating whether any rows in T2 refer to it: create view T1_v as select *, id in (select id from T2) as cannot_delete from T1; But beware! It seems that this particular form DOESN'T use the index we've just created. On the other hand, this very similarone does: create view T1_v as select *, exists (select * from T2 where id=t.id) as cannot_delete from T1 t; Now, when you create your user interface, you can just look at the cannot_delete field to see whether the delete button shouldbe enabled. This should run in logarithmic time. If this isn't fast enough you could instead make cannot_delete a real column and havetriggers on changes to T2 that change its value. But I wouldn't resort to this unless you are really desperate. Regards, --Phil.
On Tuesday 29 June 2004 07:19 pm, Phil Endecott wrote: > Kenneth Gonsalves <lawgon@thenilgiris.com> wrote: > > in my app i have a table where the id serves as a foreign key for > > one or more other tables. if i want to delete a row in the table, > > i currently search the other tables where this table is referenced > > to see if the row is in use - and then, if not in use, permit > > deletion. > > Now if i want the delete button in my app to be disabled whenever > > a row that is in use is selected, searching the database every time > > would dramatically slow down the app. > > Basically you do have to do this search. But it won't be too slow if you > create an index on the foreign key. pity. thought postgres would have some function like 'in_use' to tell when a row that is used as a foreign key is in actual use and hence cannot be deleted. surely, in a database of millions of records, it wouldnt have search them all to find if the row is in use? in my case the id field serves as a foreign key in several tables -- regards kg http://www.onlineindianhotels.net - hotel bookings reservations in over 4600 hotels in India http://www.ootygolfclub.org
On Thu, 1 Jul 2004, Kenneth Gonsalves wrote: > On Tuesday 29 June 2004 07:19 pm, Phil Endecott wrote: > > Kenneth Gonsalves <lawgon@thenilgiris.com> wrote: > > > in my app i have a table where the id serves as a foreign key for > > > one or more other tables. if i want to delete a row in the table, > > > i currently search the other tables where this table is referenced > > > to see if the row is in use - and then, if not in use, permit > > > deletion. > > > > Now if i want the delete button in my app to be disabled whenever > > > a row that is in use is selected, searching the database every time > > > would dramatically slow down the app. > > > > Basically you do have to do this search. But it won't be too slow if you > > create an index on the foreign key. > > pity. thought postgres would have some function like 'in_use' to tell when a > row that is used as a foreign key is in actual use and hence cannot be > deleted. surely, in a database of millions of records, it wouldnt have search > them all to find if the row is in use? in my case the id field serves as a Well, it's kind of a losing proposition either way. One way (the one we take right now) involves checking rows on the fktable on pk update/delete which is hoped to be fast if the user creates appropriate indexes but has issues if that plan isn't fast. The other would probably involve trying to keep track of known in_use. There are some problems there however in knowing what that value should be. For example after a delete of a row in the fktable, do you know what in_use is on its referenced row without finding all things that reference it and checking them (and what about seeing or locking stuff for concurrent transactions?) You can use a known in_use vs unknown in_use vs known not in use style value to get around that, but then I think there'd still be issues in guaranteeing the correct semantics and I think rows may often end up in unknown state at which point you scan anyway.
>>>in my app i have a table where the id serves as a foreign key for >>>one or more other tables. if i want to delete a row in the table, >>>i currently search the other tables where this table is referenced >>>to see if the row is in use - and then, if not in use, permit >>>deletion. >>>Now if i want the delete button in my app to be disabled whenever >>>a row that is in use is selected, searching the database every time >>>would dramatically slow down the app. >> >>Basically you do have to do this search. But it won't be too slow if you >>create an index on the foreign key. > > pity. thought postgres would have some function like 'in_use' to tell when a > row that is used as a foreign key is in actual use and hence cannot be > deleted. surely, in a database of millions of records, it wouldnt have search > them all to find if the row is in use? It doesn't "search them all" if you have an index. If your database has a million records it needs to look at only 20 index entries, as 2^20 is about a million. (At least that's what I, naively, think it should do - anyone who knows more want to correct me?) --Phil.
O kyrios Phil Endecott egrapse stis Jul 1, 2004 : > >>>in my app i have a table where the id serves as a foreign key for > >>>one or more other tables. if i want to delete a row in the table, > >>>i currently search the other tables where this table is referenced > >>>to see if the row is in use - and then, if not in use, permit > >>>deletion. > >>>Now if i want the delete button in my app to be disabled whenever > >>>a row that is in use is selected, searching the database every time > >>>would dramatically slow down the app. There's an alternative approach to take. Educate your users to be familiar with PostgreSQL error messages (E.g. ERROR: update or delete on "vessels" violates foreign key constraint "$1" on "certificates" ) > >> > >>Basically you do have to do this search. But it won't be too slow if you > >>create an index on the foreign key. > > > > pity. thought postgres would have some function like 'in_use' to tell when a > > row that is used as a foreign key is in actual use and hence cannot be > > deleted. surely, in a database of millions of records, it wouldnt have search > > them all to find if the row is in use? > > It doesn't "search them all" if you have an index. If your database has > a million records it needs to look at only 20 index entries, as 2^20 is > about a million. (At least that's what I, naively, think it should do - > anyone who knows more want to correct me?) > > --Phil. > > > ---------------------------(end of broadcast)--------------------------- > TIP 7: don't forget to increase your free space map settings > -- -Achilleus