From: Peter Kirk (peterkirk@qaya.org)
Date: Tue Nov 25 2003 - 06:37:56 EST
On 24/11/2003 16:56, Philippe Verdy wrote:
>Peter Kirk writes:
>
>
>>If conformance clause C10 is taken to be operable at all levels, this
>>makes a nonsense of the concept of normalisation stability within
>>databases etc.
>>
>>
>
>I don't think that the stability of normalization influence this: as long as
>there's a guarantee of being able to restore any desired normalized form,
>processes can use any canonically equivalent representation of strings.
>
>
As I understood it, stability of normalisation (as used as justification
for a refusal to correct errors in combining class allocations) is
understood as a guarantee that a process will receive a string from
another process, or from a backing store, in the same normalisation form
that in which it was sent or stored, without the need for
renormalisation. But that guarantee fails if the communication or
storage system is permitted to perform canonical transformations on the
data e.g. for compression.
If stability of normalisation means only that the required normalisation
form may be regenerated, and that data which has to be in a particular
normalisation form must always be checked and if necessary renormalised,
then the main argument for not changing combining classes disappears.
>In fact, this stability is a great benefit as it effectively gives the full
>freedom to transform strings into canonically equivalent forms if this is
>needed: if a database must be built for performance reasons with a
>particular normalization form, its interface will then be able to perform
>this normalization freely. ...
>
But only if it is able to be sure that this particular normalisation
form is stable, and cannot be transformed by a low level process hidden
from it into a different normalisation form or a canonically equivalent
but not normalised form.
>... The same is true for data compression algorithms.
>
>So it's the absence of stability which would make impossible this
>rearrangement of normalization forms...
>
>
Canonical equivalence is unaffected if combining classes are rearranged,
though not if they are split or joined. It is only the normalised forms
of strings which may be changed. So this is no argument against
rearranging combining classes.
-- Peter Kirk peter@qaya.org (personal) peterkirk@qaya.org (work) http://www.qaya.org/
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