Re: multibyte charater set in levenshtein function

Hackers,

The current version of levenshtein function in fuzzystrmatch contrib modulte
doesn't work properly with multibyte charater sets.

test=# select levenshtein('фыва','аыва');
levenshtein
-------------
2
(1 row)

My patch make this function works properly with multibyte charater sets.

test=# select levenshtein('фыва','аыва');
levenshtein
-------------
1
(1 row)

Also it avoids text_to_cstring call.

Regards,
Alexander Korotkov.

Excerpts from Alexander Korotkov's message of lun may 10 11:35:02 -0400 2010:
> Hackers,
>
> The current version of levenshtein function in fuzzystrmatch contrib modulte
> doesn't work properly with multibyte charater sets.

> My patch make this function works properly with multibyte charater sets.

Great. Please add it to the next commitfest:
http://commitfest.postgresql.org

On a quick look, I didn't like the way you separated the
"pg_database_encoding_max_length() > 1" cases. There seem to be too
much common code. Can that be refactored a bit better?
--

On Wed, May 12, 2010 at 11:04 PM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>wrote:

> On a quick look, I didn't like the way you separated the
> "pg_database_encoding_max_length() > 1" cases. There seem to be too
> much common code. Can that be refactored a bit better?
>
I did a little refactoring in order to avoid some similar code.
I'm not quite sure about my CHAR_CMP macro. Is it a good idea?

Alexander Korotkov escribió:
> On Wed, May 12, 2010 at 11:04 PM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>wrote:
>
> > On a quick look, I didn't like the way you separated the
> > "pg_database_encoding_max_length() > 1" cases. There seem to be too
> > much common code. Can that be refactored a bit better?
> >
> I did a little refactoring in order to avoid some similar code.
> I'm not quite sure about my CHAR_CMP macro. Is it a good idea?

Well, since it's only used in one place, why are you defining a macro at
all?

--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

On Thu, May 13, 2010 at 6:03 AM, Alvaro Herrera <alvherre(at)commandprompt(dot)com
> wrote:

> Well, since it's only used in one place, why are you defining a macro at
> all?
>
In order to structure code better. My question was about another. Is memcmp
function good choice to compare very short sequences of bytes (from 1 to 4
bytes)?

Hello Hackers!

I have extended my patch by introducing levenshtein_less_equal function.
This function have additional argument max_d and stops calculating when
distance exceeds max_d. With low values of max_d function works much faster
than original one.

The example of original levenshtein function usage:

test=# select word, levenshtein(word, 'consistent') as dist from words where
levenshtein(word, 'consistent') <= 2 order by dist;
word | dist
-------------+------
consistent | 0
insistent | 2
consistency | 2
coexistent | 2
consistence | 2
(5 rows)

test=# explain analyze select word, levenshtein(word, 'consistent') as dist
from words where levenshtein(word, 'consistent') <= 2 order by dist;
QUERY PLAN

---------------------------------------------------------------------------------------------------------------
Sort (cost=2779.13..2830.38 rows=20502 width=8) (actual
time=203.652..203.658 rows=5 loops=1)
Sort Key: (levenshtein(word, 'consistent'::text))
Sort Method: quicksort Memory: 25kB
-> Seq Scan on words (cost=0.00..1310.83 rows=20502 width=8) (actual
time=19.019..203.601 rows=5 loops=1)
Filter: (levenshtein(word, 'consistent'::text) <= 2)
Total runtime: 203.723 ms
(6 rows)

Example of levenshtein_less_equal usage in this case:

test=# select word, levenshtein_less_equal(word, 'consistent', 2) as dist
from words where levenshtein_less_equal(word, 'consistent', 2) <= 2 order by
dist;
word | dist
-------------+------
consistent | 0
insistent | 2
consistency | 2
coexistent | 2
consistence | 2

test=# explain analyze select word, levenshtein_less_equal(word,
'consistent', 2) as dist from words where levenshtein_less_equal(word,
'consistent', 2) <= 2 order by dist;
QUERY PLAN

-------------------------------------------------------------------------------------------------------------
Sort (cost=2779.13..2830.38 rows=20502 width=8) (actual
time=42.198..42.203 rows=5 loops=1)
Sort Key: (levenshtein_less_equal(word, 'consistent'::text, 2))
Sort Method: quicksort Memory: 25kB
-> Seq Scan on words (cost=0.00..1310.83 rows=20502 width=8) (actual
time=5.391..42.143 rows=5 loops=1)
Filter: (levenshtein_less_equal(word, 'consistent'::text, 2) <= 2)
Total runtime: 42.292 ms
(6 rows)

In the example above levenshtein_less_equal works about 5 times faster.

With best regards,
Alexander Korotkov.