Re: all foreign key should have index?

From: Dan <>
Date: Thu, 02 Feb 2006 10:14:41 -0600
Message-ID: <HdqEf.30$>

On 2/1/2006 3:08 PM, Gene Wirchenko wrote:
> On Wed, 1 Feb 2006 20:17:11 +0000 (UTC), "Murdoc"
> <> wrote:

>> Gene Wirchenko wrote:
>>> On Tue, 31 Jan 2006 22:12:56 +0000, Eric Junkermann
>>> <> wrote:
>>> [snip]
>>>> When you delete a parent row, or update its key, the DBMS needs to find 
>>>> the children, either to cascade the operation or to forbid it - how can 
>>>> it do this efficiently without an index? But of course if you never do 
>>>> those things,
>>>> you might still need it to find child rows efficiently anyway.
>>>      Why does it have to be an index?

>> Generally: efficiency. If you want the DB to enforce referential integrity, the operation of doing
>> so needs to be efficient. A search of the entire 'FK' table to ensure that a record can be deleted
>>from the 'PK' table is (a) inefficient; and (b) pointless. Or even worse, on a cascade delete. The
>> search (when using an index) really comes down to "Is there an entry in the index for this field
>> value?".
>> Again, it does come down to the size of the table. A table with a maximum of about 10 rows is not
>> going to have a large performance gain with an index (or maybe a performance detriment).

> That is not the point. Why does it have to be an INDEX?
> You have been assuming that an index is the only way. It is not.
> Sincerely,
> Gene Wirchenko

Please don't say "embed a pointer in the record..." Received on Thu Feb 02 2006 - 17:14:41 CET

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