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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
View File

@ -1078,10 +1078,13 @@ SELECT '52093.89'::money::numeric::float8;
<para> <para>
Values of type <type>character</type> are physically padded Values of type <type>character</type> are physically padded
with spaces to the specified width <replaceable>n</>, and are with spaces to the specified width <replaceable>n</>, and are
stored and displayed that way. However, the padding spaces are stored and displayed that way. However, trailing spaces are treated as
treated as semantically insignificant. Trailing spaces are semantically insignificant and disregarded when comparing two values
disregarded when comparing two values of type <type>character</type>, of type <type>character</type>. In collations where whitespace
and they will be removed when converting a <type>character</type> value 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 to one of the other string types. Note that trailing spaces
<emphasis>are</> semantically significant in <emphasis>are</> semantically significant in
<type>character varying</type> and <type>text</type> values, and <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; len2;
int cmp; 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); len1 = bcTruelen(arg1);
len2 = bcTruelen(arg2); len2 = bcTruelen(arg2);