Обсуждение: tsearch strategy for incremental search

I am trying to implement an incremental search engine.  The service
should start searching when the user has typed at least 3 characters.
I am thinking of using the following strategy:

a) Create a function string_to_three_char_tsvector(str text) that
would generate the tsvector composed of the three-letter lexemes that
begin all the admissible words within the parameter str.
b) Using this function, create an indexed tsvector column:  three_char_index.
c) Given the query string query_string (assume query_string containing
at least 3 characters):
SELECT *
FROM mytable, plainto_tsquery((string_to_three_char_tsvector(query_string))::text)
AS query
WHERE three_char_index @@ query
AND text_field LIKE '%' || str || '%';

Once I've narrowed the field of possibilities down to the correct
3-letter lexemes, there are fewer than 100 lines to search through
with LIKE.  I could even repeat the exercise with 4-letter lexemes if
these numbers were to grow or if I needed the extra boost in
performance.

So, two questions to postgres/tsearch experts:

1) Does that seem like a decent overall strategy?

2) About the function string_to_three_char_tsvector(text), I cannot
think of an elegant way of writing this.  Is it possible to do better
than the following:

str => cast to tsvector => cast to text => for each lexeme-string,
take first-three-char substring => concat back together => cast to
tsvector

Is there a nice way of performing the middle operation?  Like
splitting the string to an array...

2008/6/30 Oleg Bartunov <oleg@sai.msu.su>:
> tsearch will have prefix search support in 8.4.

Thanks Oleg!  That's fantastic news!

In the meantime, carrying on with my earlier idea, here's the little
function I came up with for extracting the 3-char-lexeme tsvector, in
case anyone's interested:

CREATE OR REPLACE FUNCTION public.to_three_char_tsvector(str text)
  RETURNS tsvector AS
$BODY$declare
somerow record;
shortened_text text := '';
BEGIN
FOR somerow IN SELECT regexp_split_to_table(trim(both '\'' from
strip(to_tsvector('simple',str))::text),'\'') AS item LOOP
    shortened_text := shortened_text || ' ' ||
COALESCE(substring(somerow.item for 3), '');
END LOOP;
RETURN strip(to_tsvector(shortened_text));
END;$BODY$
  LANGUAGE 'plpgsql' IMMUTABLE;

Pierre,

you, probably, can use custom configuration, which uses pg_3chars
dictionary.

Oleg
On Mon, 30 Jun 2008, Pierre Thibaudeau wrote:

> 2008/6/30 Oleg Bartunov <oleg@sai.msu.su>:
>> tsearch will have prefix search support in 8.4.
>
> Thanks Oleg!  That's fantastic news!
>
> In the meantime, carrying on with my earlier idea, here's the little
> function I came up with for extracting the 3-char-lexeme tsvector, in
> case anyone's interested:
>
> CREATE OR REPLACE FUNCTION public.to_three_char_tsvector(str text)
>  RETURNS tsvector AS
> $BODY$declare
> somerow record;
> shortened_text text := '';
> BEGIN
> FOR somerow IN SELECT regexp_split_to_table(trim(both '\'' from
> strip(to_tsvector('simple',str))::text),'\'') AS item LOOP
>     shortened_text := shortened_text || ' ' ||
> COALESCE(substring(somerow.item for 3), '');
> END LOOP;
> RETURN strip(to_tsvector(shortened_text));
> END;$BODY$
>  LANGUAGE 'plpgsql' IMMUTABLE;
>
>

     Regards,
         Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83

2008/6/30 Oleg Bartunov <oleg@sai.msu.su>:
> you, probably, can use custom configuration, which uses pg_3chars
> dictionary.

Thanks Oleg.  That sounds like a cleverer (and more natural) idea than mine.

I am intrigued:  is "pg_3chars" something that's part of the current
distribution of postgresql (I did a quick search and found nothing),
or are you implying that it would be worth writing a tsearch
configuration along the lines of my earlier ideas?

On Mon, 30 Jun 2008, Pierre Thibaudeau wrote:

> 2008/6/30 Oleg Bartunov <oleg@sai.msu.su>:
>> you, probably, can use custom configuration, which uses pg_3chars
>> dictionary.
>
> Thanks Oleg.  That sounds like a cleverer (and more natural) idea than mine.
>
> I am intrigued:  is "pg_3chars" something that's part of the current
> distribution of postgresql (I did a quick search and found nothing),
> or are you implying that it would be worth writing a tsearch
> configuration along the lines of my earlier ideas?

it's just a suggestion name :) btw, if you're really lazy you can
use dict_regex (http://vo.astronet.ru/arxiv/dict_regex.html) dictionary.

     Regards,
         Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83