Re: tsearch2: ts_headline performance problem

Anders,

headline is a slow, since it should read and process an original document,
so you should avoid simple search query, which calculates ts_headline()
for each document in the result set. Use subselect instead !

In example below, 2nd query is way faster the 1rd !

select id,headline(body,q),rank(ti,q) as rank
from apod, to_tsquery('stars') q
where ti @@ q order by rank desc limit 10;

and

select id,headline(body,q),rank
from ( select id,body,q, rank(ti,q) as rank from apod, to_tsquery('stars') q
where ti @@ q order by rank desc limit 10) as foo;

Oleg
On Sun, 6 Jan 2008, Anders ?stergaard Jensen wrote:

> Hi there people,
>
> I'm trying to build a text search engine for relatively large PDF documents
> with a web frontend. I use PostgreSQL 8.3 beta3/4 with tsearch2 for full-text
> indexing. Everything performs quite well, until I start indexing 800 pages of
> text into the database (with the utf8 danish locale and dictionary) -- then
> ts_headline performs unreasonably slow. I use a GIN index on the table
> (called document_revisions which is linked to a master document table called
> 'documents' -- the system is made for version tracking of multiple documents
> into the same index) and an update trigger for maintaining the tsvector row
> on the table.
>
> Omitting ts_headline yields the following EXPLAIN result:
>
> <snip>
> metabase=# explain analyze select * from f_search_revision('website');
> NOTICE: f_search_revisions_arr: loop: (1) doc_id = (1000)
> CONTEXT: SQL statement "SELECT f_search_revision_arr( $1 ) AS res"
> PL/pgSQL function "f_search_revision_int" line 13 at SQL statement
> SQL function "f_search_revision" statement 1
> NOTICE: hest: ({1000})
> CONTEXT: SQL function "f_search_revision" statement 1
> NOTICE: total ranking for (1) eq (0.0865452) for id = (1000)
> CONTEXT: SQL function "f_search_revision" statement 1
> QUERY PLAN
> -----------------------------------------------------------------------------------------------------------------------------
> Function Scan on f_search_revision (cost=0.00..260.00 rows=1000 width=408)
> (actual time=2413.384..2413.384 rows=1 loops=1)
> Total runtime: 2413.465 ms
> (2 rows)
> </snip>
>
> The function f_search_revision invokes another function that fetches all
> matching document_revisions without calling ts_headline (method
> f_search_revision_arr), followed by a post-processing of the matching rows
> for fetching the ts_ranking values and ts_headline. The following snippet
> shows the query plan for the function call with the same query:
>
> <snip>
> metabase=# explain analyze select * from f_search_revision_arr('website');
> NOTICE: f_search_revisions_arr: loop: (1) doc_id = (1000)
> QUERY PLAN
> ---------------------------------------------------------------------------------------------------------------------
> Function Scan on f_search_revision_arr (cost=0.00..0.26 rows=1 width=32)
> (actual time=3.417..3.418 rows=1 loops=1)
> Total runtime: 3.442 ms
> (2 rows)
> </snip>
>
> Here is the code for the functions that I am using:
>
> CREATE OR REPLACE FUNCTION f_search_revision_arr(q TEXT) RETURNS INTEGER[] AS
> $$
> declare
> iter record; results integer[];
> i integer;
> begin
> i := 1;
> for iter in SELECT * FROM documents doc, plainto_tsquery(q)
> AS tsq
> WHERE (doc.search_idx @@ tsq)
> OR doc.id IN (SELECT document_id FROM
> document_revisions dr, plainto_tsquery(q) AS tsq2
> WHERE dr.search_idx @@ tsq2) loop
> results[i] = iter.id;
> raise notice 'f_search_revisions_arr: loop: (%)
> doc_id = (%)', i, results[i];
> i := i + 1;
> end loop;
> return results;
> end;
> $$ LANGUAGE plpgsql;
>
> CREATE OR REPLACE FUNCTION f_search_revision_int(q TEXT) RETURNS
> t_search_result[] AS $$
> declare
> doc_ids integer[];
> doc_rankings float[];
> i integer;
> iter record;
> pointer_doc record;
> pointer_rev record;
> pointer_type record;
> results t_search_result[];
> begin
> i := 1;
>
> SELECT INTO iter f_search_revision_arr(q) AS res;
> doc_ids := iter.res;
> raise notice 'hest: (%)', doc_ids;
>
> -- Ensure that something was found:
> if (doc_ids IS NULL) then
> return results;
> end if;
>
> for i in 1..array_upper(doc_ids, 1) loop
> -- raise notice 'upper = (%)', doc_ids[i];
> SELECT INTO pointer_doc ts_rank(doc.search_idx, tsq)
> AS rank_doc,
> ts_headline(doc.name, tsq) AS headline_name,
> ts_headline(doc.description, tsq) AS
> headline_description,
> ts_headline(doc.keywords, tsq) AS
> headline_keywords,
> doc.name,
> doc.description,
> doc.keywords,
> doc.document_type_id
> FROM documents doc, plainto_tsquery(q) AS tsq
> WHERE doc.id = doc_ids[i];
>
> SELECT INTO pointer_rev ts_rank(rev.search_idx, tsq)
> AS rank_rev,
> ts_headline(rev.content, tsq) AS
> headline_content,
> rev.id AS revision_id
> FROM document_revisions rev,
> plainto_tsquery(q) AS tsq
> WHERE rev.document_id = doc_ids[i];
>
> -- Select the document type:
> SELECT INTO pointer_type mime_type, type_name,
> image_url, extension, internal_type
> FROM document_types WHERE id =
> pointer_doc.document_type_id;
>
> doc_rankings[i] = pointer_doc.rank_doc +
> pointer_rev.rank_rev; -- SUM!
> raise notice 'total ranking for (%) eq (%) for id =
> (%)', i, doc_rankings[i], doc_ids[i];
> results[i] = ROW(pointer_rev.revision_id,
> doc_ids[i],
> pointer_doc.name,
> pointer_doc.description, pointer_doc.keywords,
> pointer_doc.document_type_id,
> pointer_type.mime_type, pointer_type.type_name, pointer_type.image_url,
> pointer_type.extension, pointer_type.internal_type,
> doc_rankings[i],
> pointer_doc.headline_name, pointer_doc.headline_description,
> pointer_doc.headline_keywords, pointer_rev.headline_content,
> 0);
> end loop;
> return results;
> end;
> $$ LANGUAGE plpgsql;
>
> CREATE OR REPLACE FUNCTION f_search_revision(q TEXT) RETURNS SETOF
> t_search_result AS $$
> SELECT * FROM f_unnest(f_search_revision_int($1));
> $$ LANGUAGE sql;
>
> ------
>
> How come that ts_headline yields so bad performance? Is there any way of
> tuning ts_headline? I need the highlighting functionality for presenting the
> search results in the web frontend properly. Alternately, are there any
> alternatives to the ts_headline function?
>
> Sincerely,
>
> Anders
>
>
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster

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