TSearch2: find a QUERY that does match a single document

On Fri, 12 Sep 2008, Dmitry Koterov wrote:

Hello.

TSearch2 allows to search a table of tsvectors by a single tsquery.
I need to solve the reverse problem.

*I have a large table of tsquery. I need to find all tsqueries in that table
that match a single document tsvector:
*
CREATE TABLE "test"."test_tsq" (
"id" SERIAL,
"q" TSQUERY NOT NULL,
CONSTRAINT "test_tsq_pkey" PRIMARY KEY("id")
);

insert into test.test_tsq(q)
select to_tsquery(g || 'x' || g) from generate_series(100000, 900000) as g;

explain analyze
select * from test.test_tsq
where to_tsvector('400000x400000') @@ q

why do you need tsvector @@ q ? Much better to use tsquery = tsquery

test=# explain analyze select * from test_tsq where q = '400000x400000'::tsque>
QUERY PLAN
-----------------------------------------------------------------------------------------------------------
Seq Scan on test_tsq (cost=0.00..16667.01 rows=1 width=38) (actual time=129.208..341.111 rows=1 loops=1)
Filter: (q = '''400000x400000'''::tsquery)
Total runtime: 341.134 ms
(3 rows)

Time: 341.478 ms

This gets a strange explain analyze:

QUERY PLAN
Seq Scan on test_tsq (cost=0.00..17477.01 rows=800 width=36) (actual
time=68.698..181.458 rows=1 loops=1)
Filter: ('''400000x400000'':1'::tsvector @@ q)
Total runtime: 181.484 ms

No matter if I use GIST index on test_tsq.q or not: the explain analyze
result is the same.
So, why "rows=800"? The table contains much more rows...

'800' is the number of estimated rows, which is not good, since you got only
1 row.

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

explain analyze

select * from test.test_tsq
where to_tsvector('400000x400000') @@ q

why do you need tsvector @@ q ? Much better to use tsquery = tsquery

test=# explain analyze select * from test_tsq where q =
'400000x400000'::tsque>
QUERY PLAN

-----------------------------------------------------------------------------------------------------------
Seq Scan on test_tsq (cost=0.00..16667.01 rows=1 width=38) (actual
time=129.208..341.111 rows=1 loops=1)
Filter: (q = '''400000x400000'''::tsquery)
Total runtime: 341.134 ms
(3 rows)

M-mmm... Seems your understood me incorrectly.

I have to find NOT queries which are exactly equal to another query, BUT
queries which MATCH the GIVEN document. '400000x400000' was a sample only,
in real cases it will be 1-2K document.

Here is a more realistic sample:

explain analyze
select * from test.test_tsq
where to_tsvector('
Here is a real document text. It may be long, 1-2K.
In this sample it contains a lexem "400000x400000", so there is a tsquery
in test_tsq.q which matches this document. I need to find all such queries
fast.
Of course, in real cases the document text is unpredictable.
') @@ q

QUERY PLAN

Seq Scan on test_tsq (cost=0.00..17477.01 rows=800 width=36) (actual
time=68.698..181.458 rows=1 loops=1)
Filter: ('''400000x400000'':1'::tsvector @@ q)
Total runtime: 181.484 ms

'800' is the number of estimated rows, which is not good, since you got
only 1 row.

Why 800? The table contains 800000 rows, and seqscan is used. Does it scan
the whole table or not? If yes, possibly there is a bug in explain output?
(No mater if I create GIST index on test_tsq.q or not, the number of rows is
still 800, so it seems to me that GIST index is not used at all.)