Normalisation stability, was: Compression through normalization

[Peter Kirk]

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
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