Re: fast DISTINCT or EXIST

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От Arjen van der Meijden
Тема Re: fast DISTINCT or EXIST
Дата
Msg-id 46178F65.5090807@tweakers.net
обсуждение исходный текст
Ответ на fast DISTINCT or EXIST  (Tilo Buschmann)
Ответы Re: fast DISTINCT or EXIST  (Tom Lane)
Список pgsql-performance
Дерево обсуждения
fast DISTINCT or EXIST  (Tilo Buschmann, )
 Re: fast DISTINCT or EXIST  (Arjen van der Meijden, )
  Re: fast DISTINCT or EXIST  (Tom Lane, )
   Re: fast DISTINCT or EXIST  (Tilo Buschmann, )
    Re: fast DISTINCT or EXIST  (Tom Lane, )
    Re: fast DISTINCT or EXIST  (Arjen van der Meijden, )
Can't you use something like this? Or is the distinct on the t.cd_id
still causing the major slowdown here?

SELECT ... FROM cd
   JOIN tracks ...
WHERE cd.id IN (SELECT DISTINCT t.cd_id FROM tracks t
      WHERE t.tstitle @@ plainto_tsquery('simple','education') LIMIT 10)

If that is your main culprit, you could also use two limits based on the
fact that there will be at most X songs per cd which would match your
title (my not very educated guess is 3x). Its a bit ugly... but if that
is what it takes to make postgresql not scan your entire index, so be it...

SELECT ... FROM cd
   JOIN tracks ...
WHERE cd.id IN (SELECT DISTINCT cd_id FROM (SELECT t.cd_id FROM tracks t
      WHERE t.tstitle @@ plainto_tsquery('simple','education') LIMIT 30)
as foo LIMIT 10)


Best regards,

Arjen

On 7-4-2007 12:47 Tilo Buschmann wrote:
> Hello,
>
> I am trying to build a application to search CDs and their tracks and I
> am experiencing some performance difficulties.
>
> The database is very simple at the moment, two tables "cd" and "tracks"
> contain the CD-information and their respective tracks. A column
> "cd_id" in public.tracks is the foreign key to the cd table.
>
> #v+
>                                   Table "public.cd"
>    Column    |       Type        |                     Modifiers
> -------------+-------------------+----------------------------------------------------
>  revision    | integer           | not null default 0
>  disc_length | integer           |
>  via         | character varying |
>  cd_id       | integer           | not null default nextval('cd_cd_id_seq'::regclass)
>  discid      | integer           | not null
>  title       | character varying | not null
>  artist      | character varying | not null
>  year        | smallint          |
>  genre       | character varying |
>  ext         | character varying |
>  tstitle     | tsvector          |
>  tsartist    | tsvector          |
> Indexes:
>     "cd_id_key" PRIMARY KEY, btree (cd_id)
>     "discid_key" UNIQUE, btree (discid)
>     "tsartist_cd_idx" gist (tsartist)
>     "tstitle_cd_idx" gist (tstitle)
> Check constraints:
>     "year_check" CHECK ("year" IS NULL OR "year" >= 0 AND "year" <= 10000)
> Tablespace: "d_separate"
>
>                                    Table "public.tracks"
>   Column  |       Type        |                         Modifiers
> ----------+-------------------+-----------------------------------------------------------
>  track_id | integer           | not null default nextval('tracks_track_id_seq'::regclass)
>  cd_id    | integer           | not null
>  title    | character varying |
>  artist   | character varying |
>  ext      | character varying |
>  length   | integer           |
>  number   | smallint          | not null default 0
>  tstitle  | tsvector          |
>  tsartist | tsvector          |
> Indexes:
>     "tracks_pkey" PRIMARY KEY, btree (track_id)
>     "cdid_tracks_idx" btree (cd_id)
>     "tsartist_tracks_idx" gist (tsartist)
>     "tstitle_tracks_idx" gin (tstitle)
> Foreign-key constraints:
>     "tracks_cd_id_fkey" FOREIGN KEY (cd_id) REFERENCES cd(cd_id) ON UPDATE RESTRICT ON DELETE RESTRICT
> Tablespace: "d_separate"
>
> #v-
>
> I am using tsearch2 to be able to search very fast for CD and track
> artists and titles.
>
> The database is created only once and I expect SELECTS to happen very
> often, therefore the indexes will not hurt the performance. I also ran
> a VACUUM FULL ANALYSE.
>
> The query that I want to optimise at the moment is the "Give me all CDs
> with their tracks, that contain a track with the Title 'foobar'". The
> query is very expensive, so I try to limit it to 10 cds at once.
>
> My first idea was:
>
> #+
> cddb=# EXPLAIN ANALYSE SELECT cd.cd_id,cd.title,cd.artist,tracks.title FROM tracks JOIN (SELECT
cd.cd_id,cd.artist,cd.titleFROM cd JOIN tracks USING (cd_id) WHERE tracks.tstitle @@
plainto_tsquery('simple','education')LIMIT 10) AS cd USING (cd_id); 
>                                                                         QUERY PLAN
                                    
>
----------------------------------------------------------------------------------------------------------------------------------------------------------
>  Nested Loop  (cost=0.00..3852.42 rows=11974 width=91) (actual time=310.983..972.739 rows=136 loops=1)
>    ->  Limit  (cost=0.00..121.94 rows=10 width=46) (actual time=264.797..650.178 rows=10 loops=1)
>          ->  Nested Loop  (cost=0.00..227602.43 rows=18665 width=46) (actual time=264.793..650.165 rows=10 loops=1)
>                ->  Index Scan using tstitle_tracks_idx on tracks  (cost=0.00..73402.74 rows=18665 width=4) (actual
time=155.516..155.578rows=10 loops=1) 
>                      Index Cond: (tstitle @@ '''education'''::tsquery)
>                ->  Index Scan using cd_id_key on cd  (cost=0.00..8.25 rows=1 width=46) (actual time=49.452..49.453
rows=1loops=10) 
>                      Index Cond: (public.cd.cd_id = public.tracks.cd_id)
>    ->  Index Scan using cdid_tracks_idx on tracks  (cost=0.00..358.08 rows=1197 width=27) (actual time=29.588..32.239
rows=14loops=10) 
>          Index Cond: (public.tracks.cd_id = cd.cd_id)
>  Total runtime: 972.917 ms
> (10 rows)
> #v-
>
>
> The query is fast enough, but erroneous. If a cd contains more than one
> track, that matches the condition, the inner SELECT will return more
> than one cd and therefore the whole query will shield duplicate cds.
>
> The solution is to either insert DISTINCT into the above query or use
> EXISTS as condition, but both queries show a terrible performance:
>
> #v+
> cddb=# EXPLAIN ANALYSE SELECT cd.cd_id,cd.title,cd.artist,tracks.title FROM tracks JOIN (SELECT DISTINCT
cd.cd_id,cd.artist,cd.titleFROM cd JOIN tracks USING (cd_id) WHERE tracks.tstitle @@
plainto_tsquery('simple','education')LIMIT 10) AS cd USING (cd_id); 
>                                                                            QUERY PLAN
>
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
>  Nested Loop  (cost=152390.12..156120.71 rows=11974 width=91) (actual time=37356.517..37605.073 rows=137 loops=1)
>    ->  Limit  (cost=152390.12..152390.22 rows=10 width=46) (actual time=37289.598..37289.643 rows=10 loops=1)
>          ->  Unique  (cost=152390.12..152576.77 rows=18665 width=46) (actual time=37289.594..37289.629 rows=10
loops=1)
>                ->  Sort  (cost=152390.12..152436.79 rows=18665 width=46) (actual time=37289.590..37289.601 rows=12
loops=1)
>                      Sort Key: public.cd.cd_id, public.cd.artist, public.cd.title
>                      ->  Hash Join  (cost=78926.50..151066.02 rows=18665 width=46) (actual time=36214.504..37285.974
rows=811loops=1) 
>                            Hash Cond: (public.tracks.cd_id = public.cd.cd_id)
>                            ->  Bitmap Heap Scan on tracks  (cost=536.76..59707.31 rows=18665 width=4) (actual
time=0.724..39.253rows=811 loops=1) 
>                                  Recheck Cond: (tstitle @@ '''education'''::tsquery)
>                                  ->  Bitmap Index Scan on tstitle_tracks_idx  (cost=0.00..532.09 rows=18665 width=0)
(actualtime=0.492..0.492 rows=811 loops=1) 
>                                        Index Cond: (tstitle @@ '''education'''::tsquery)
>                            ->  Hash  (cost=49111.33..49111.33 rows=1344433 width=46) (actual
time=36211.598..36211.598rows=1344433 loops=1) 
>                                  ->  Seq Scan on cd  (cost=0.00..49111.33 rows=1344433 width=46) (actual
time=31.094..19813.716rows=1344433 loops=1) 
>    ->  Index Scan using cdid_tracks_idx on tracks  (cost=0.00..358.08 rows=1197 width=27) (actual time=31.294..31.527
rows=14loops=10) 
>          Index Cond: (public.tracks.cd_id = cd.cd_id)
>  Total runtime: 37614.523 ms
> (16 rows)
>
> cddb=# EXPLAIN ANALYSE SELECT cd.cd_id,cd.artist,cd.title,tracks.title FROM tracks JOIN (SELECT
cd.cd_id,cd.artist,cd.titleFROM cd WHERE EXISTS (SELECT 1 FROM tracks WHERE tracks.cd_id = cd.cd_id AND tracks.tstitle
@@plainto_tsquery('simple','education')) LIMIT 10) as cd USING (cd_id); 
>                                                                     QUERY PLAN
                            
>
--------------------------------------------------------------------------------------------------------------------------------------------------
>  Nested Loop  (cost=0.00..10023.37 rows=11974 width=91) (actual time=126.607..40853.563 rows=148 loops=1)
>    ->  Limit  (cost=0.00..6292.89 rows=10 width=46) (actual time=126.587..40853.072 rows=10 loops=1)
>          ->  Seq Scan on cd  (cost=0.00..423018283.46 rows=672216 width=46) (actual time=126.584..40853.035 rows=10
loops=1)
>                Filter: (subplan)
>                SubPlan
>                  ->  Index Scan using cdid_tracks_idx on tracks  (cost=0.00..314.61 rows=1 width=0) (actual
time=1.025..1.025rows=0 loops=39706) 
>                        Index Cond: (cd_id = $0)
>                        Filter: (tstitle @@ '''education'''::tsquery)
>    ->  Index Scan using cdid_tracks_idx on tracks  (cost=0.00..358.08 rows=1197 width=27) (actual time=0.011..0.029
rows=15loops=10) 
>          Index Cond: (tracks.cd_id = cd.cd_id)
>  Total runtime: 40853.789 ms
> (11 rows)
> #v-
>
> Rephrasing the EXISTS-query as an IN-query did not help the
> performance, either.
>
> I get the impression, that I am blind and cannot find the obvious
> solution, do you have any idea how to accomplish, what I am trying?
>
> Best Regards,
>
> Tilo
>
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster
>

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