Re: [SQL] Comparison semantics of CHAR data type

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.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +