Note in-line
--
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk
The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
April 2004 Iceland
June 2004 UK - Optimising Oracle Seminar
"Paul Drake" <drak0nian_at_yahoo.com> wrote in message
news:1ac7c7b3.0403141841.617f46c_at_posting.google.com...
>
> Steve suggested that Oracle's hash algorithm was not sufficient in
> uniqueness, and that a primary key (such as a sequence) would provide
> a better source for the hash.
>
> In one of the block dumps, he showed a case of where the hash was not
> unique, which unsets a flag which represents that all rows in the
> block are unique. If that flag is set, oracle knows that it has a
> unique mapping between the hash and the row.
>
That covers all the points he showed us in Denmark.
> This is on a block by block basis, so a single non-unique mapping of
> hash value to row is not catastrophic.
>
That statement is a little misleading. If there is no collision
(i.e. there is a unique mapping) then the row access is a
single latched 'examination'. If the flag is set to show that
a collision has occurred, the row has to be acquired with
a normal double-latched get and full block scan.
I haven't quantified the difference (yet) and the circumstances
where this would make a difference worth worrying about,
but it is likely to double (at least) the resource requirements on
each access to that block.
> thanks again for the excellent presentations.
>
My pleasure - the Hotsos conference is probably the
most exciting one I do all year, so I'm pleased to be
there.
Received on Mon Mar 15 2004 - 02:18:53 CST
