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 <br...@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ Everyone has their own god. +
diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
new file mode 100644
index 30fd9bb..9635004
*** a/doc/src/sgml/datatype.sgml
--- b/doc/src/sgml/datatype.sgml
*************** SELECT '52093.89'::money::numeric::float
*** 1072,1081 ****
<para>
Values of type <type>character</type> are physically padded
with spaces to the specified width <replaceable>n</>, and are
! stored and displayed that way. However, the padding spaces are
! treated as semantically insignificant. Trailing spaces are
! disregarded when comparing two values of type <type>character</type>,
! and they will be removed when converting a <type>character</type> value
to one of the other string types. Note that trailing spaces
<emphasis>are</> semantically significant in
<type>character varying</type> and <type>text</type> values, and
--- 1072,1084 ----
<para>
Values of type <type>character</type> are physically padded
with spaces to the specified width <replaceable>n</>, and are
! stored and displayed that way. However, trailing spaces are treated as
! semantically insignificant and disregarded when comparing two values
! of type <type>character</type>. In collations where whitespace
! is significant, this behavior can produce unexpected results,
! e.g. <command>SELECT 'a '::CHAR(2) collate "C" < 'a\n'::CHAR(2)
! returns true.
! Trailing spaces are removed when converting a <type>character</type> value
to one of the other string types. Note that trailing spaces
<emphasis>are</> semantically significant in
<type>character varying</type> and <type>text</type> values, and
diff --git a/src/backend/utils/adt/varchar.c b/src/backend/utils/adt/varchar.c
new file mode 100644
index 284b5d1..502ca44
*** a/src/backend/utils/adt/varchar.c
--- b/src/backend/utils/adt/varchar.c
*************** bpcharcmp(PG_FUNCTION_ARGS)
*** 846,863 ****
len2;
int cmp;
- /*
- * Trimming trailing spaces off of both strings can cause a string
- * with a character less than a space to compare greater than a
- * space-extended string, e.g. this returns false:
- * SELECT E'ab\n'::CHAR(10) < E'ab '::CHAR(10);
- * even though '\n' is less than the space if CHAR(10) was
- * space-extended. The correct solution would be to trim only
- * the longer string to be the same length of the shorter, if
- * possible, then do the comparison. However, changing this
- * might break existing indexes, breaking binary upgrades.
- * For details, see http://www.postgresql.org/message-id/CAK+WP1xdmyswEehMuetNztM4H199Z1w9KWRHVMKzyyFM+hV=z...@mail.gmail.com
- */
len1 = bcTruelen(arg1);
len2 = bcTruelen(arg2);
--- 846,851 ----
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