Re: [SQL] Comparison semantics of CHAR data type

On Fri, Feb 14, 2014 at 05:02:26PM -0500, Bruce Momjian wrote:
> On Thu, Feb 13, 2014 at 09:47:01PM -0500, Bruce Momjian wrote:
> > On Wed, Oct 16, 2013 at 02:17:11PM -0400, Bruce Momjian wrote:
> > > > > You can see the UTF8 case is fine because \n is considered greater
> > > > > than space, but in the C locale, where \n is less than space, the
> > > > > false return value shows the problem with
> > > > > internal_bpchar_pattern_compare() trimming the string and first
> > > > > comparing on lengths.  This is exactly the problem you outline, where
> > > > > space trimming assumes everything is less than a space.
> > > > 
> > > > For collations other than C some of those issues that have to do with
> > > > string comparisons might simply be hidden, depending on how strcoll()
> > > > handles inputs off different lengths: If strcoll() applies implicit
> > > > space padding to the shorter value, there won't be any visible
> > > > difference in ordering between bpchar and varchar values.  If strcoll()
> > > > does not apply such space padding, the right-trimming of bpchar values
> > > > causes very similar issues even in a en_US collation.
> > 
> > I have added the attached C comment to explain the problem, and added a
> > TODO item to fix it if we ever break binary upgrading.
> > 
> > Does anyone think this warrants a doc mention?
> 
> I have done some more thinking on this and I found a way to document
> this, which reduces our need to actually fix it some day.  I am afraid
> the behavioral change needed to fix this might break so many
> applications that the fix will never be done, though I will keep the
> TODO item until I get more feedback on that.  Patch attached.

Patch applied.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
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