Thread: BUG #18857: Abnormal string comparison results

The following bug has been logged on the website:

Bug reference:      18857
Logged by:          dlyixue Xu
Email address:      2322370369@qq.com
PostgreSQL version: 17.3
Operating system:   Ubuntu 22.04
Description:

When I use a conditional query, I find that it returns results that are not
as expected. After checking, I found that there is a problem with string
comparison here.
As shown in the example below, on the ASCII table ‘?’ is larger than ‘&’. I
don't think ‘t’ should be returned here. In the MySQL database it does
return 0.
Is there some special consideration here?

postgres=# SELECT ('?' <= '&*CcCV5');
 ?column? 
----------
 t
(1 row)

PG Bug reporting form <noreply@postgresql.org> writes:
> As shown in the example below, on the ASCII table ‘?’ is larger than ‘&’. I
> don't think ‘t’ should be returned here. In the MySQL database it does
> return 0.
> Is there some special consideration here?

Read about collations at

https://www.postgresql.org/docs/current/collation.html

You are probably using a non-C collation.  You could say

postgres=# SELECT ('?' <= '&*CcCV5' collate "C");
 ?column?
----------
 f
(1 row)

but more likely you want to switch it at the database level.

            regards, tom lane

On Thu, Mar 20, 2025 at 09:53:32AM +0000, PG Bug reporting form wrote:
> The following bug has been logged on the website:
> 
> Bug reference:      18857
> Logged by:          dlyixue Xu
> Email address:      2322370369@qq.com
> PostgreSQL version: 17.3
> Operating system:   Ubuntu 22.04
> Description:        
> 
> When I use a conditional query, I find that it returns results that are not
> as expected. After checking, I found that there is a problem with string
> comparison here.
> As shown in the example below, on the ASCII table ‘?’ is larger than ‘&’. I
> don't think ‘t’ should be returned here. In the MySQL database it does
> return 0.
> Is there some special consideration here?
> 
> postgres=# SELECT ('?' <= '&*CcCV5');
>  ?column? 
> ----------
>  t

I think your collation is ignoring punctuation:

    SELECT '?Z' <= '&A';
     ?column?
    ----------
     f
    
    SELECT '?A' <= '&Z';
     ?column?
    ----------
     t
    
    SELECT '?' <= '&*CcCV5' COLLATE "C";
     ?column?
    ----------
     f

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Do not let urgent matters crowd out time for investment in the future.