fast DISTINCT or EXIST - Mailing list pgsql-performance

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..36211.598
rows=1344433loops=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