In message
<5e5ef002669716488cf5f69288ba6b093bf...@msgl-lon01-es01.msgluk.local>,
Raymond P. de Bourbon <rdebour...@msgl.com> writes
Does anyone have any experience using Sequentially hashed files? The
documentation on Sequentially hashed files is not very explicit about
what kinds of benefits or problems can be expected / experienced.. It
also only hints towards one use case, but IMO doesn't do enough to
clearly define when these types of files are actually beneficial and
why...
I've never had any need to use sequentially hashed files, but I'm quite
happy to try and explain it ...
Two questions to start with:
Firstly, is your app generating sequential numeric keys? And do you only
create records, never delete them?
Secondly (less importantly), are your records of a roughly consistent
size?
The point of sequential files is that, if your answer to both questions
is yes, a sequential file is trivially and perfectly balanced. And such
files are common enough to make creating a special file type worth
doing.
Rather than trying to find some hash algorithm that does a good job,
sequential files use the key value as the hash value. So if each record
fills a group, record 1 goes in group 1, record 2 goes in group 2, etc
etc. If two records fill a group, then it goes 1, 1:2, 1:3,2, 1:3,2:4,
etc etc.
I would have thought that if the typical record ended up in overflow,
then sequential would be a good match for sequential keys too regardless
of whether the size is consistent. However, DON'T take my GUESS on that.
And bear in mind the advice is, if the answer to the first question is
"no", then DON'T go near sequential files. They're a special case, for a
special-case situation. It's just that that "special case" is actually
quite common.
Cheers,
Wol
--
Anthony W. Youngman <pi...@thewolery.demon.co.uk>
'Yings, yow graley yin! Suz ae rikt dheu,' said the blue man, taking the
thimble. 'What *is* he?' said Magrat. 'They're gnomes,' said Nanny. The man
lowered the thimble. 'Pictsies!' Carpe Jugulum, Terry Pratchett 1998
Visit the MaVerick web-site - <http://www.maverick-dbms.org> Open Source Pick
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