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? -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + Everyone has their own god. +
diff --git a/src/backend/utils/adt/varchar.c b/src/backend/utils/adt/varchar.c new file mode 100644 index 502ca44..284b5d1 *** a/src/backend/utils/adt/varchar.c --- b/src/backend/utils/adt/varchar.c *************** bpcharcmp(PG_FUNCTION_ARGS) *** 846,851 **** --- 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);
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