Обсуждение: Re: char(n) to varchar or text conversion should strip

> >> Hmm ... now that's an interesting thought.  So the input converter would
> >> actively strip trailing blanks, output would add them back,
>
> > But how would the output know how many to put back?
>
> The output routine would need access to the column typmod.  Which it
> would have, in simple "SELECT columnname" cases, but this is a serious
> weakness of the scheme in general.

I think it is not really a weakness of the scheme, but a weakness that typmod
is not available in some places where it would actually be needed.
One effect of this is, that all the varlena datatypes have a redundant
length info per row, even for such types that have the same length for
all rows of one table (e.g. numeric(8,2), and char(n)). In a lot of cases
that means 50-100% more disk space than actually needed.

I can see, that making typmod availabe in more places would probably
be major work (or not feasible at all), but I think it would generally be
of (great) value.

To the problem of concatenation:
Would it be feasible to alter the concatenation method to concatenate
the results of the output functions of the relevant expressions ?
Imho that would actually return the expected results more often than it
currently does, and it would fix the typmod issue for char(n) concatenation.

Andreas

"Zeugswetter Andreas SB SD" <ZeugswetterA@spardat.at> writes:
> I think it is not really a weakness of the scheme, but a weakness that typmod
> is not available in some places where it would actually be needed. 
> One effect of this is, that all the varlena datatypes have a redundant 
> length info per row, even for such types that have the same length for 
> all rows of one table (e.g. numeric(8,2), and char(n)).

That argument doesn't hold any water, seeing that neither of your
examples are actually fixed-width... numeric doesn't store leading
or trailing zeroes, and char(n) is not a fixed-width item when dealing
with multibyte encodings.  And on top of that, there's TOAST to
think about.  So I don't think there's any shot at getting rid of the
varlen word.

> Would it be feasible to alter the concatenation method to concatenate 
> the results of the output functions of the relevant expressions ?
> Imho that would actually return the expected results more often than it 
> currently does, and it would fix the typmod issue for char(n) concatenation.

No, it wouldn't really fix the typmod issue; you still have the problem
of where is the output function going to get the typmod from?  If the
concatenation argument is anything but a simple column reference, you've
still got trouble.
        regards, tom lane