Re: Searching a gin index

James,

syntax is documented on
http://www.postgresql.org/docs/8.3/static/textsearch-tables.html#TEXTSEARCH-TABLES-SEARCH
and in the Introduction http://www.postgresql.org/docs/8.3/static/textsearch-intro.html#TEXTSEARCH-MATCHING
text-search operator was specified for tsvector @@ tsquery.
You did wrong twice, you didn't specified type tsvector and you forgot about coalesce.

There is general rule for partial indexes - you should use the same expression in query as you used in create index command.

Oleg
On Fri, 6 Feb 2009, James Dooley wrote:

Oleg, but I am only interested in whether or not the syntax of my
search-query is correct.

Having created the index as I mentioned above, would the correct way of
searching and using that index be

... AND (title || '' || description || '' || name) @@ plainto_tsquery('car')

or should it be as Richard just mentioned

... AND to_tsvector(title || '' || description || '' || name) @@
plainto_tsquery('car')

or some other way ?



On Fri, Feb 6, 2009 at 3:30 PM, Richard Huxton <dev@archonet.com> wrote:

James Dooley wrote:

Hi again,

I have set my configuration as default and I have created a GIN index on
three columns, like this

create index textsearch_index on products using gin(strip( to_tsvector(
'my_config', title || '' || description || '' || name)))

Searching these columns the way I have

... AND (title || '' || description || '' || name) @@

plainto_tsquery('car')

seems not to be correct since it's taking as much time as non-indexed.

PG's planner isn't smart enough to transform a complex expression so as
to use a functional index (which is what you've got). You need to
mention the function explicitly.

So, if you had:
 CREATE INDEX lowercase_idx ON mytable ( lower(mycolumn) )
You then need to search against "lower(mycolumn)" and not just expect
the planner to notice that mycolumn="abc" could use the index.

The following should work for you as a starting point:

CREATE TABLE tsearch_tbl (id SERIAL, title text, body text,
PRIMARY KEY (id));

INSERT INTO tsearch_tbl (title, body)
SELECT 'title number ' || n, 'This is body number ' || n
FROM generate_series(1,9999) n;

ANALYSE tsearch_tbl;

CREATE INDEX tsearch_tbl_words_idx ON tsearch_tbl USING gin (
to_tsvector('english', title || body) );

EXPLAIN ANALYSE SELECT * FROM tsearch_tbl WHERE to_tsvector('english',
title || body) @@ to_tsquery('17');
                                                        QUERY PLAN


-------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on tsearch_tbl  (cost=4.34..34.76 rows=10 width=45)
(actual time=0.067..0.067 rows=1 loops=1)
 Recheck Cond: (to_tsvector('english'::regconfig, (title || body)) @@
to_tsquery('17'::text))
 ->  Bitmap Index Scan on tsearch_tbl_words_idx  (cost=0.00..4.34
rows=10 width=0) (actual time=0.059..0.059 rows=1 loops=1)
       Index Cond: (to_tsvector('english'::regconfig, (title || body))
@@ to_tsquery('17'::text))
 Total runtime: 0.121 ms


Note that you'll have problems if any of your text-fields contain nulls
(since null || anything = null).

Personally, unless I'm dealing with a large table, I like to have a
separate tsvector column which I keep up to date with triggers. It makes
it easier to debug problems.

--
 Richard Huxton
 Archonet Ltd

       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