Re: Support LIKE with nondeterministic collations

On 15.11.24 05:26, jian he wrote:
> /*
> * Now build a substring of the text and try to match it against
> * the subpattern. t is the start of the text, t1 is one past the
> * last byte. We start with a zero-length string.
> */
> t1 = t
> t1len = tlen;
> for (;;)
> {
> int cmp;
> CHECK_FOR_INTERRUPTS();
> cmp = pg_strncoll(subpat, subpatlen, t, (t1 - t), locale);
>
> select '.foo.' LIKE '_oo' COLLATE ign_punct;
> pg_strncoll's iteration of the first 4 argument values.
> oo 2 foo. 0
> oo 2 foo. 1
> oo 2 foo. 2
> oo 2 foo. 3
> oo 2 foo. 4
>
> seems there is a shortcut/optimization.
> if subpat don't have wildcard(percent sign, underscore)
> then we can have less pg_strncoll calls?

How would you do that? You need to try all combinations to find one
that matches.

> minimum case to trigger error within GenericMatchText
> since no related tests.
> create table t1(a text collate case_insensitive, b text collate "C");
> insert into t1 values ('a','a');
> select a like b from t1;

This results in

ERROR: 42P22: could not determine which collation to use for LIKE
HINT: Use the COLLATE clause to set the collation explicitly.

which is the expected behavior.

> at 9.7.1. LIKE section, we still don't know what "wildcard" is.
> we mentioned it at 9.7.2.
> maybe we can add a sentence at the end of:
> <para>
> If <replaceable>pattern</replaceable> does not contain percent
> signs or underscores, then the pattern only represents the string
> itself; in that case <function>LIKE</function> acts like the
> equals operator. An underscore (<literal>_</literal>) in
> <replaceable>pattern</replaceable> stands for (matches) any single
> character; a percent sign (<literal>%</literal>) matches any sequence
> of zero or more characters.
> </para>
>
> saying underscore and percent sign are wildcards in LIKE.
> other than that, I can understand the doc.

Ok, I agree that could be clarified.