I found a little LIKE/CHAR() surprise --- below is a table and query
against a CHAR(10) field:
test=> CREATE TABLE test (x char(10));
CREATE TABLE
test=> INSERT INTO test values ('hi');
INSERT 0 1
test=> SELECT * FROM test WHERE x = 'hi';
x
------------
hi
(1 row)
The above works because both sides are converted to 'bpchar'; explain
shows that:
test=> EXPLAIN SELECT * FROM test WHERE x = 'hi';
QUERY PLAN
------------------------------------------------------
Seq Scan on test (cost=0.00..33.12 rows=9 width=14)
Filter: (x = 'hi'::bpchar)
^^^^^^
(2 rows)
The following does not work:
test=> SELECT * FROM test WHERE x LIKE 'hi';
x
---
(0 rows)
It seems LIKE is considering the trailing CHAR(10) field spaces as
significant, even though our documentations says:
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.
It says trailing spaces are not significant for character comparisons
--- the real question is whether LIKE is a comparison. Obvioiusly '='
is a comparison, but the system does not treat LIKE as a comparison in
terms of trailing spaces. Is that desired behavior?
I did an EXPLAIN on the query and found '~~' was being used and 'hi' was
being converted to text:
test=> explain select * from test where x like 'hi';
QUERY PLAN
------------------------------------------------------
Seq Scan on test (cost=0.00..33.12 rows=9 width=14)
Filter: (x ~~ 'hi'::text)
^^ ^^^^
(2 rows)
so I then checked psql \do to see what operators there were for ~~:
test=> \do ~~
List of operators
Schema | Name | Left arg type | Right arg type | Result type |
Description
------------+------+---------------+----------------+-------------+-------------------------
pg_catalog | ~~ | bytea | bytea | boolean |
matches LIKE expression
--> pg_catalog | ~~ | character | text | boolean |
matches LIKE expression
pg_catalog | ~~ | name | text | boolean |
matches LIKE expression
pg_catalog | ~~ | text | text | boolean |
matches LIKE expression
(4 rows)
The one marked matches the arguments so it seems the comparison being
done is not character and character, but character and text.
I realize trim() could be used to get the desired behavior, but is our
behavior consistent?
--
Bruce Momjian <[email protected]> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
--
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