Обсуждение: BUG #4398: Backslashes get escaped despite of backslash_quote=off

The following bug has been logged online:

Bug reference:      4398
Logged by:          Rainer
Email address:      rainer@hamburg.ccc.de
PostgreSQL version: 8.3.3 /b1400
Operating system:   Windows XP
Description:        Backslashes get escaped despite of backslash_quote=off
Details:

Subject to: Backslashes get escaped in like operations despite of
backslash_quote=off
psql \version: PostgreSQL 8.3.3, compiled by Visual C++ build 1400

Goal to achieve: SQL compliant handling of escaping which means...
Expected result: ...backslashes are not to be escaped, single quotes are to
be escaped by itself

Current configuration is: backslash_quote=off and
standard_conforming_strings=on

Problem/Bug is: Operator "=" works fine, operator "like" does not (see test
case below)

Status is: I checked the three concerning manual chapters (4.1.2.1., 9.7.1.,
18.12.1.) which promise another result than actually received by the
following test case:

To see for yourself please feel free to use the following short script to
create a small test database with a table and a single test record:

-- --- BOF ---

create database dbs_test_escaping with encoding 'LATIN9';

\connect dbs_test_escaping

create table tbl_test_escaping
(
  test_id int4 not null,
  test_name varchar(100),
  test_comment text,
  constraint prk_test_id primary key (test_id)
);

-- this should make the comment actually to be like this: test "\te%s_t
<\'\>
insert into tbl_test_escaping
(test_id, test_name, test_comment)
values (1, 'Testname', 'test "\te%s_t <\''\>');

-- --- EOF ---

Now check the following statements whose results seem not to reflect the
above configuration:

1. Select the record by a single equal (works fine):
select * from tbl_test_escaping where test_comment = 'test "\te%s_t
<\''\>';

2. Select the record by a like-statement (won't return any result):
select * from tbl_test_escaping where test_comment like '%<\''\>%';

3. Select the record by a like-statement which is manually escaped and has
the escape flag reset (works fine):
select * from tbl_test_escaping where test_comment like '%<\''\>%' escape
'';

4. Select the record by a like-statement which is manually escaped (should
not match because of backslash_quote=off but it matches):
select * from tbl_test_escaping where test_comment like '%<\\''\\>%';

Two questions:
1. What I actually want: Shouldn't the second statement work by
documentation without the escape flag?
2. What I do not understand: Why does the fourth statement return a result
as backslash_quote is off?


Thanks sincerely,
Rainer
rainer@hamburg.ccc.de

"Rainer" <rainer@hamburg.ccc.de> writes:
> Description:        Backslashes get escaped despite of backslash_quote=off

Aren't you looking for standard_conforming_strings?  backslash_quote is
something else entirely, and doesn't actually do anything at all when
backslash escaping is disabled.

> Two questions:
> 1. What I actually want: Shouldn't the second statement work by
> documentation without the escape flag?

No.  standard_conforming_strings has nothing to do with the behavior of
LIKE (nor does backslash_quote).  They just control the initial parsing
of SQL string literals.

> 2. What I do not understand: Why does the fourth statement return a result
> as backslash_quote is off?

It looks like a perfectly good match to me.

            regards, tom lane

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Hi Tom,
Hi list,

(I'm the same as rainer@hamburg.ccc.de who started this thread,
this is my regular address, you may compare the gpg-keys if in doubt :)

On 4 Sep 2008 at 11:51, Tom Lane wrote:
>
> Aren't you looking for standard_conforming_strings?

Of course I did and I mentioned my configuration as:
backslash_quote=off and standard_conforming_strings=on

So it should be as close to the SQL standard as ever possible.


> backslash_quote is something else entirely, and doesn't actually
> do anything at all when backslash escaping is disabled.

Yes, this was my preferred behaviour at all. But I don't see a way:
How can backslashes be stopped from escaping themselves in like-statements
if not by setting both params as "backslash_quote=off" and
"standard_conforming_strings=on" and when not using the "escape" flag?


> > 2. What I do not understand: Why does the fourth statement return a result
> > as backslash_quote is off?
>
> It looks like a perfectly good match to me.

Hmm, I really would have expected the same behaviour of backslashes
between an equal comparison and a like comparison as backslashes
according to your view and to my conviction are irrelevant in that case.

In short:
The following statements always should return the same
result in a table of just one row, but they do not.

this returns a row:
  select * from tbl_test_escaping
    where test_comment = 'test "\te%s_t<\''\>';

this does not return a row:
  select * from tbl_test_escaping
    where test_comment like '%<\''\>%';

Is this very different behaviour in string handling really intentional?

To have to escape backslashes in _another_ way depending on "=" or "like"
even if conforming strings is on and any special backslash handling is off,
sounds actually very strange and rather unwanted to me.

And what makes me wonder is that I can make it work by adding the escape
flag only in like statement, which I do not need in the case of an equal:

this works fine, too:
  select * from tbl_test_escaping where
    test_comment like '%<\''\>%' escape '';

I'm just looking for any way of configuration where I get this behaviour
but have not to add the escape '' -flag in every single like-statement.

Regards,
Rainer


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"Rainer Glueck" <rainer.glueck@netminds.de> writes:
> How can backslashes be stopped from escaping themselves in like-statements

Use ESCAPE ''.  There isn't any other way provided.  I suppose we might
consider inventing a separate GUC parameter to specify what the default
escape sequence is, but we wouldn't overload any existing parameter to
do it.

I think the reason the issue hasn't come up before is that it doesn't
seem real useful: if you have no escape character then you have no
way to quote % and _, and those are at least as likely to need to be
plain data characters as \ is.

            regards, tom lane

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Hello Tom,

On 4 Sep 2008 at 12:53, Tom Lane wrote:
>
> "Rainer Glueck" <rainer.glueck@netminds.de> writes:
> > How can backslashes be stopped from escaping themselves in like-statements
>
> Use ESCAPE ''.  There isn't any other way provided.
> ...
> if you have no escape character then you have no way to quote % and _

Yes, that's true indeed.
And since I learned that the escape clause is recommended to cope
with that matter not only by PostgreSQL, but also by Transact-SQL,
I'm going to adapt my database connector this way, withdraw this topic
and thank you for pushing me a little bit more to enlightenment. :)

Regards,
Rainer




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