Message-ID: <40E3E003.4050405@chezphil.org>
Date: Thu, 01 Jul 2004 10:57:23 +0100
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To: Kenneth Gonsalves <lawgon@thenilgiris.com>,
	pgsql-sql@postgresql.org
Subject: Re: finding if a foreign key is in use
References: <0406290618410W.02481@thenilgiris.com> <7679232@chezphil.org>
	<0407010743031D.02481@thenilgiris.com>
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From: Phil Endecott <spam_from_postgresql_sql@chezphil.org>

>>>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.