Error in char(n) example in documentation
От | Peter Geoghegan |
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Тема | Error in char(n) example in documentation |
Дата | |
Msg-id | CAM3SWZRaaD+1ercd3szpGKeA_PMTOp7gvK5sQe-9n7gAb19U1A@mail.gmail.com обсуждение исходный текст |
Ответы |
Re: Error in char(n) example in documentation
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Список | pgsql-hackers |
I think that this example in the docs [1] is wrong: """ Values of type character are physically padded with spaces to the specified width n, and are stored and displayed that way. However, trailing spaces are treated as semantically insignificant and disregarded when comparing two values of type character. In collations where whitespace is significant, this behavior can produce unexpected results, e.g. SELECT 'a '::CHAR(2) collate "C" < 'a\n'::CHAR(2) returns true. """ Now, it is the case that the SQL statement will yield true: postgres=# SELECT 'a '::CHAR(2) collate "C" < 'a\n'::CHAR(2); ?column? ---------- t (1 row) However, so does the same formulation with varchar, even though this example is motivated by showing how char(n) differs from other character types like varchar(n): postgres=# SELECT 'a '::VARCHAR(2) collate "C" < 'a\n'::VARCHAR(2); ?column? ---------- t (1 row) I believe that the problem is that commit 8457d0bec did not correctly transcribe the string constant with C-like escape notation from a C code comment that it removed as redundant (it was roughly the same example -- guess the simplification of the example went too far). Attached patch fixes the documentation. When an "E" is added so the C-like escape is correctly interpreted, the char(n) example continues to yield true: postgres=# SELECT 'a '::CHAR(2) collate "C" < E'a\n'::CHAR(2); ?column? ---------- t (1 row) But changing the cast to varchar will now yield a different result (perhaps the intuitive result to those not familiar with char(n) semantics): postgres=# SELECT 'a '::VARCHAR(2) collate "C" < E'a\n'::VARCHAR(2); ?column? ---------- f (1 row) [1] http://www.postgresql.org/docs/devel/static/datatype-character.html -- Peter Geoghegan
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