Re: 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.
>>
>>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.