Databasen - Revisited

Matt Sergeant msergeant at
Wed Oct 18 19:20:35 BST 2006

On 18-Oct-06, at 1:08 PM, Peter Corlett wrote:

> On Wed, Oct 18, 2006 at 12:16:00PM -0400, Matt Sergeant wrote:
> [...]
>> There are lots of fairly obvious right answers (localisation would  
>> be one
>> obvious one), but most people won't get the fact that the primary key
>> index is a bad thing, because the entire table fits into a single  
>> page of
>> pretty much every DB I know, so index access is actually slower  
>> than a
>> full table scan.
> I think I'd be wary of working for somebody who insisted this was  
> actually
> better and considered this sort of hack a virtue.

Sorry, but when it comes down to performance, squeezing out every  
last drop is not a hack. Do you consider a schwarzian transform a  
hack too?

> Any RDBMS worth its salt will skip the index if it's clearly faster  
> to do a
> full table scan.

You prefer to force the optimiser to have to examine its choices than  
give it none? There's a reason databases provide extended syntaxes to  
ignore the optimiser choices and force use of an index - optimisers  
aren't that smart. (and yes, this is another one of the reasons why I  
think ORMs are bad).

It was a very specific example to try and get at very specific  
knowledge (fitting a table into a page). If you can think of a better  
way to get at that knowledge please let me know.

> So all you've actually achieved is a less robust schema
> which may, ironically, cause the optimiser to make poorer decisions.

Please state when this would happen unless it's just postulating.


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