Re: Support LIKE with nondeterministic collations - Mailing list pgsql-hackers

On 27.07.24 00:32, Paul A Jungwirth wrote:
> On Thu, Jun 27, 2024 at 11:31 PM Peter Eisentraut <peter@eisentraut.org> wrote:
>> Here is an updated patch for this.
> 
> I took a look at this. I added some tests and found a few that give
> the wrong result (I believe). The new tests are included in the
> attached patch, along with the results I expect. Here are the
> failures:

Thanks, these are great test cases.

> 
>   -- inner %% matches b then zero:
>   SELECT U&'cb\0061\0308' LIKE U&'c%%\00E4' COLLATE ignore_accents;
>    ?column?
>   ----------
> - t
> + f
>   (1 row)
> 
>   -- trailing _ matches two codepoints that form one char:
>   SELECT U&'cb\0061\0308' LIKE U&'cb_' COLLATE ignore_accents;
>    ?column?
>   ----------
> - t
> + f
>   (1 row)
> 
> -- leading % matches zero:
>   SELECT U&'\0061\0308bc' LIKE U&'%\00E4bc' COLLATE ignore_accents;
>    ?column?
>   ----------
> - t
> + f
>   (1 row)
> 
>   -- leading % matches zero (with later %):
>   SELECT U&'\0061\0308bc' LIKE U&'%\00E4%c' COLLATE ignore_accents;
>    ?column?
>   ----------
> - t
> + f
>   (1 row)
> 
> I think the 1st, 3rd, and 4th failures are all from % not backtracking
> to match zero chars.

These are all because of this in like_match.c:

        * Otherwise, scan for a text position at which we can match the
        * rest of the pattern.  The first remaining pattern char is known
        * to be a regular or escaped literal character, so we can compare
        * the first pattern byte to each text byte to avoid recursing
        * more than we have to.  [...]

This shortcut doesn't work with nondeterministic collations, so we have 
to recurse in any case here.  I have fixed that in the new patch; these 
test cases work now.

> The 2nd failure I'm not sure about. Maybe my expectation is wrong, but
> then why does the same test pass with __ leading not trailing? Surely
> they should be consistent.

The question is why is

SELECT U&'cb\0061\0308' LIKE U&'cb_' COLLATE ignore_accents;  -- false

but

SELECT U&'\0061\0308bc' LIKE U&'_bc' COLLATE ignore_accents;  -- true

The second one matches because

     SELECT U&'\0308bc' = 'bc' COLLATE ignore_accents;

So the accent character will be ignored if it's adjacent to another 
fixed substring in the pattern.