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docs: document behavior of CHAR() comparisons with chars < space

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Space trimming rather than space-padding causes unusual behavior, which
might not be standards-compliant.

Also remove recently-added now-redundant C comment.
This commit is contained in:
Bruce Momjian 2014-02-24 12:09:23 -05:00
parent 6615e77439
commit 8457d0beca
2 changed files with 7 additions and 16 deletions
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11
doc/src/sgml/datatype.sgml
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@ -1078,10 +1078,13 @@ SELECT '52093.89'::money::numeric::float8;
<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
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

12
src/backend/utils/adt/varchar.c
View File

@ -846,18 +846,6 @@ bpcharcmp(PG_FUNCTION_ARGS)
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=zA@mail.gmail.com
*/
len1 = bcTruelen(arg1);
len2 = bcTruelen(arg2);