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From Michael Segel <mse...@segel.com>
Subject Re: "generated by default" question
Date Fri, 16 Jun 2006 02:10:16 GMT
On Thursday 15 June 2006 8:52 pm, Daniel Noll wrote:
Sigh.

If you're doing an insert on a table with an identity column, then yes I 
believe that the index would be in memory. Or a memory page that was cached.

But you're missing the point.
When do you get the max() value of the index? ;-)
And why do you need it? ;-)

...

The point is that you don't need it.
If the insert has a value X for the identity column and X > CSV, then set CSV 
= X;

It works every time.

> Michael Segel wrote:
> >> Presumably he column would be indexed if you're doing operations like
> >> max() on it anyway, so efficiency isn't the problem.  I suspect the
> >> problem would be what to do when two users tried to insert rows at
> >> around the same time.  The second would fail to commit the transaction
> >> and have to try again.
> >>
> >> Daniel
> >
> > No.
> > First, you're still returning a value from the sequence that will cause
> > an exception.  So  you have overhead. Also you'd have a lot harder time
> > trying to manage the inserts. Also, you have to consider that its cheaper
> > to get a value from memory than it is to apply the max() function on a
> > value from the index.
>
> Ah.  I was making the assumption that the indexes were implemented
> "properly."  For instance, an index should keep a certain amount in
> memory in order to have a reasonable chance of running quickly.  So the
> max and min should already be in-memory.
>
> I take it that Derby doesn't do this?
>
> Daniel

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