It appears that the ORDER BY rank operation is the slowing factor.
If I remove it then the query is pretty fast. Is there another way
to perform ORDER BY such that it does not do a sort?
Benjamin
On Oct 5, 2007, at 3:57 PM, Benjamin Arai wrote:
On Oct 5, 2007, at 8:32 AM, Oleg Bartunov wrote:
On Fri, 5 Oct 2007, Tom Lane wrote:
Benjamin Arai <benjamin@xxxxxxxxxxxx> writes:
# explain analyze select * FROM fulltext_article, to_tsquery
('simple','dog') AS q WHERE idxfti @@ q ORDER BY rank(idxfti,
q) DESC;
QUERY PLAN
-------------------------------------------------------------------
-----
-------------------------------------------------------------------
-----
------------
Sort (cost=6576.74..6579.07 rows=933 width=774) (actual
time=12969.237..12970.490 rows=5119 loops=1)
Sort Key: rank(fulltext_article.idxfti, q.q)
-> Nested Loop (cost=3069.79..6530.71 rows=933 width=774)
(actual time=209.513..12955.498 rows=5119 loops=1)
-> Function Scan on q (cost=0.00..0.01 rows=1 width=32)
(actual time=0.005..0.006 rows=1 loops=1)
-> Bitmap Heap Scan on fulltext_article
(cost=3069.79..6516.70 rows=933 width=742) (actual
time=209.322..234.390 rows=5119 loops=1)
Recheck Cond: (fulltext_article.idxfti @@ q.q)
-> Bitmap Index Scan on
fulltext_article_idxfti_idx
(cost=0.00..3069.56 rows=933 width=0) (actual time=208.373..208.373
rows=5119 loops=1)
Index Cond: (fulltext_article.idxfti @@ q.q)
Total runtime: 12973.035 ms
(9 rows)
The time seems all spent at the join step, which is odd because it
really hasn't got much to do. AFAICS all it has to do is compute
the
rank() values that the sort step will use. Is it possible that
rank() is really slow?
can you try rank_cd() instead ?
Using Rank:
-# ('simple','dog') AS q WHERE idxfti @@ q ORDER BY rank(idxfti,
q) DESC;
QUERY PLAN
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------
Sort (cost=6576.74..6579.07 rows=933 width=774) (actual
time=98083.081..98084.351 rows=5119 loops=1)
Sort Key: rank(fulltext_article.idxfti, q.q)
-> Nested Loop (cost=3069.79..6530.71 rows=933 width=774)
(actual time=479.122..98067.594 rows=5119 loops=1)
-> Function Scan on q (cost=0.00..0.01 rows=1 width=32)
(actual time=0.003..0.004 rows=1 loops=1)
-> Bitmap Heap Scan on fulltext_article
(cost=3069.79..6516.70 rows=933 width=742) (actual
time=341.739..37112.110 rows=5119 loops=1)
Recheck Cond: (fulltext_article.idxfti @@ q.q)
-> Bitmap Index Scan on
fulltext_article_idxfti_idx (cost=0.00..3069.56 rows=933 width=0)
(actual time=321.443..321.443 rows=5119 loops=1)
Index Cond: (fulltext_article.idxfti @@ q.q)
Total runtime: 98087.575 ms
(9 rows)
Using Rank_cd:
# explain analyze select * FROM fulltext_article, to_tsquery
('simple','cat') AS q WHERE idxfti @@ q ORDER BY rank_cd(idxfti,
q) DESC;
QUERY PLAN
----------------------------------------------------------------------
----------------------------------------------------------------------
-----------------
Sort (cost=6576.74..6579.07 rows=933 width=774) (actual
time=199316.648..199324.631 rows=26054 loops=1)
Sort Key: rank_cd(fulltext_article.idxfti, q.q)
-> Nested Loop (cost=3069.79..6530.71 rows=933 width=774)
(actual time=871.428..199244.330 rows=26054 loops=1)
-> Function Scan on q (cost=0.00..0.01 rows=1 width=32)
(actual time=0.006..0.007 rows=1 loops=1)
-> Bitmap Heap Scan on fulltext_article
(cost=3069.79..6516.70 rows=933 width=742) (actual
time=850.674..50146.477 rows=26054 loops=1)
Recheck Cond: (fulltext_article.idxfti @@ q.q)
-> Bitmap Index Scan on
fulltext_article_idxfti_idx (cost=0.00..3069.56 rows=933 width=0)
(actual time=838.120..838.120 rows=26054 loops=1)
Index Cond: (fulltext_article.idxfti @@ q.q)
Total runtime: 199338.297 ms
(9 rows)
regards, tom lane
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Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet
(www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg@xxxxxxxxxx, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83
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