Re: pg_trgm partial-match

Hi!

On Thu, Nov 15, 2012 at 11:39 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:

> Note that we cannot do a partial-match if KEEPONLYALNUM is disabled,
> i.e., if query key contains multibyte characters. In this case, byte
> length of
> the trigram string might be larger than three, and its CRC is used as a
> trigram key instead of the trigram string itself. Because of using CRC, we
> cannot do a partial-match. Attached patch extends pg_trgm so that it
> compares a partial-match query key only when KEEPONLYALNUM is
> enabled.
>

Didn't get this point. How does KEEPONLYALNUM guarantee that each trigram
character is singlebyte?

CREATE TABLE test (val TEXT);
INSERT INTO test VALUES ('aa'), ('aaa'), ('шaaш');
CREATE INDEX trgm_idx ON test USING gin (val gin_trgm_ops);
ANALYZE test;
test=# SELECT * FROM test WHERE val LIKE '%aa%';
val
------
aa
aaa
шaaш
(3 rows)
test=# set enable_seqscan = off;
SET
test=# SELECT * FROM test WHERE val LIKE '%aa%';
val
-----
aa
aaa
(2 rows)

I think we can use partial match only for singlebyte encodings. Or, at
most, in cases when all alpha-numeric characters are singlebyte (have no
idea how to check this).

------
With best regards,
Alexander Korotkov.