Re: finding if a foreign key is in use

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.
>
>
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--
-Achilleus