Dann Corbit wrote:
> > But isn't collating sequence related to ordering? How does this
> relate
> > to padding?
>
> Right. Collating sequence is how ordering is defined. But when you
> compare two character types, they are supposed to pad according to the
> collating sequence. So whether you blank fill or pad with some special
> character when performing a comparison is defined by the collating
> sequence and not by the character type. Since we see (for instance)
> that bpchar(n) and varchar(n) pad differently when performing a
> comparison, we must assume that they have a different collating
> sequence. So the question is "what is it?"
>
> It is always possible that I have misread the standard.
OK, I understand now. It is tempting to think that the difference
between char() and varchar() is that internally they use a different
collating sequences, but that isn't the case. If it were, space would
be ignored during comparisons any place in the string, when in fact, is
it is only trailing space that char() ignores, e.g.:test=> SELECT 'a '::CHAR(10) = 'a'::CHAR(10); ?column?----------
t(1row)test=> SELECT 'a '::VARCHAR(10) = 'a'::VARCHAR(10); ?column?---------- f(1 row)test=> SELECT 'a'::CHAR(10) = '
a'::CHAR(10);?column?---------- f(1 row)test=> SELECT 'a'::VARCHAR(10) = ' a'::VARCHAR(10); ?column?---------- f(1
row)
Our docs already have:
http://candle.pha.pa.us/main/writings/pgsql/sgml/datatype-character.htmlValues of type character are physically padded
withspaces to thespecified width n, and are stored and displayed that way. However, thepadding spaces are treated as
semanticallyinsignificant. Trailingspaces are disregarded when comparing two values of type character, andthey will be
removedwhen converting a character value to one of theother string types. Note that trailing spaces are
semanticallysignificantin character varying and text values.
What additional documentation is needed?
-- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610)
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