Operatore PostgreSQL IN con prestazioni insufficienti delle subquery

Question

Perché l'operatore "IN" è lento quando utilizzato con la sottoquery?

select * 
from view1 
where id in (1,2,3,4,5,6,7,8,9,10) 
order by somedata; 

eseguito in 9 ms.

select * 
from view1 
where id in (select ext_id 
      from aggregate_table 
      order by somedata limit 10) 
order by somedata; 

esegue in 25000ms e sembra utilizzare la scansione sequenziale sulla vista (view1) invece di scansione indice chiavi primarie restituiti dalla subquery come fa in prima interrogazione.

La sottoquery select ext_id from aggregate_table order by somedata limit 10 esegue in 0,1 ms

così la lentezza della seconda query è causato da scansione sequenziale su view1 che è una vista contenente tre UNIONS e circa tre join in ogni UNION. La prima UNION contiene circa 1 milione di righe, altre molto meno. Si unisce a tabelle con alcune righe da 100K. Non è così rilevante, però, volevo solo capire il comportamento dell'operatore IN.

Quello che sto cercando di ottenere è di prendere il risultato della subquery (un set di chiavi primarie) e selezionare i dati da una vista complessa (view1) usando solo loro.

anche io non posso usare

select v1.* 
from view1 v1, 
    aggregate_table at 
where v1.id = at.ext_id 
order by at.somedata 
limit 10 

perché non voglio ordinare la grande join da somedata. Voglio solo selezionare 10 risultati dalla vista con le chiavi primarie e quindi ordinare solo questi.

La domanda è: perché l'operatore IN esegue rapidamente quando elencho esplicitamente queste chiavi e così lento quando utilizzo una subquery veloce che restituisce lo stesso set di chiavi?

EXPLAIN ANALYZE come richiesto

prima interrogazione - select * from view1 where id in (1,2,3,4,5,6,7,8,9,10) order by somedata;

Sort (cost=348.480..348.550 rows=30 width=943) (actual time=14.385..14.399 rows=10 loops=1) 
    Sort Key: "india".three 
    Sort Method: quicksort Memory: 30kB 
    -> Append (cost=47.650..347.440 rows=30 width=334) (actual time=11.528..14.275 rows=10 loops=1) 
     -> Subquery Scan "*SELECT* 1" (cost=47.650..172.110 rows=10 width=496) (actual time=11.526..12.301 rows=10 loops=1) 
       -> Nested Loop (cost=47.650..172.010 rows=10 width=496) (actual time=11.520..12.268 rows=10 loops=1) 
        -> Hash Join (cost=47.650..87.710 rows=10 width=371) (actual time=11.054..11.461 rows=10 loops=1) 
          Hash Cond: (hotel.alpha_five = juliet_xray.alpha_five) 
          -> Bitmap Heap Scan on sierra hotel (cost=42.890..82.800 rows=10 width=345) (actual time=10.835..11.203 rows=10 loops=1) 
            Recheck Cond: (four = ANY ('quebec'::integer[])) 
           -> Bitmap Index Scan on seven (cost=0.000..42.890 rows=10 width=0) (actual time=0.194..0.194 rows=10 loops=1) 
             Index Cond: (four = ANY ('quebec'::integer[])) 
          -> Hash (cost=4.340..4.340 rows=34 width=30) (actual time=0.184..0.184 rows=34 loops=1) 
           -> Seq Scan on six juliet_xray (cost=0.000..4.340 rows=34 width=30) (actual time=0.029..0.124 rows=34 loops=1) 
        -> Index Scan using charlie on juliet_two zulu (cost=0.000..8.390 rows=1 width=129) (actual time=0.065..0.067 rows=1 loops=10) 
          Index Cond: (zulu.four = hotel.victor_whiskey) 
     -> Subquery Scan "*SELECT* 2" (cost=4.760..97.420 rows=10 width=366) (actual time=0.168..0.168 rows=0 loops=1) 
       -> Hash Join (cost=4.760..97.320 rows=10 width=366) (actual time=0.165..0.165 rows=0 loops=1) 
         Hash Cond: (alpha_xray.alpha_five = juliet_xray2.alpha_five) 
        -> Nested Loop (cost=0.000..92.390 rows=10 width=340) (actual time=0.162..0.162 rows=0 loops=1) 
          -> Seq Scan on lima_echo alpha_xray (cost=0.000..8.340 rows=10 width=216) (actual time=0.159..0.159 rows=0 loops=1) 
            Filter: (four = ANY ('quebec'::integer[])) 
          -> Index Scan using charlie on juliet_two xray (cost=0.000..8.390 rows=1 width=128) (never executed) 
            Index Cond: (zulu2.four = alpha_xray.victor_whiskey) 
        -> Hash (cost=4.340..4.340 rows=34 width=30) (never executed) 
          -> Seq Scan on six uniform (cost=0.000..4.340 rows=34 width=30) (never executed) 
     -> Subquery Scan "*SELECT* 3" (cost=43.350..77.910 rows=10 width=141) (actual time=1.775..1.775 rows=0 loops=1) 
       -> Hash Join (cost=43.350..77.810 rows=10 width=141) (actual time=1.771..1.771 rows=0 loops=1) 
         Hash Cond: (golf.alpha_five = juliet_xray3.alpha_five) 
        -> Bitmap Heap Scan on lima_golf golf (cost=38.590..72.910 rows=10 width=115) (actual time=0.110..0.110 rows=0 loops=1) 
          Recheck Cond: (four = ANY ('quebec'::integer[])) 
          -> Bitmap Index Scan on victor_hotel (cost=0.000..38.590 rows=10 width=0) (actual time=0.105..0.105 rows=0 loops=1) 
            Index Cond: (four = ANY ('quebec'::integer[])) 
        -> Hash (cost=4.340..4.340 rows=34 width=30) (actual time=0.118..0.118 rows=34 loops=1) 
          -> Seq Scan on six victor_kilo (cost=0.000..4.340 rows=34 width=30) (actual time=0.007..0.063 rows=34 loops=1) 
Total runtime: 14.728 ms 

seconda query - select * from view1 where id in (select ext_id from aggregate_table order by somedata limit 10) order by somedata;

Sort (cost=254515.780..254654.090 rows=55325 width=943) (actual time=24687.475..24687.488 rows=10 loops=1) 
    Sort Key: "five".xray_alpha 
    Sort Method: quicksort Memory: 30kB 
    -> Hash Semi Join (cost=54300.820..250157.370 rows=55325 width=943) (actual time=11921.783..24687.308 rows=10 loops=1) 
      Hash Cond: ("five".lima = "delta_echo".lima) 
     -> Append (cost=54298.270..235569.720 rows=1106504 width=494) (actual time=3412.453..23091.938 rows=1106503 loops=1) 
       -> Subquery Scan "*SELECT* 1" (cost=54298.270..234227.250 rows=1100622 width=496) (actual time=3412.450..20234.122 rows=1100622 loops=1) 
        -> Hash Join (cost=54298.270..223221.030 rows=1100622 width=496) (actual time=3412.445..17078.021 rows=1100622 loops=1) 
          Hash Cond: (three_victor.xray_hotel = delta_yankee.xray_hotel) 
          -> Hash Join (cost=54293.500..180567.160 rows=1100622 width=470) (actual time=3412.251..12108.676 rows=1100622 loops=1) 
            Hash Cond: (three_victor.tango_three = quebec_seven.lima) 
           -> Seq Scan on india three_victor (cost=0.000..104261.220 rows=1100622 width=345) (actual time=0.015..3437.722 rows=1100622 loops=1) 
           -> Hash (cost=44613.780..44613.780 rows=774378 width=129) (actual time=3412.031..3412.031 rows=774603 loops=1) 
             -> Seq Scan on oscar quebec_seven (cost=0.000..44613.780 rows=774378 width=129) (actual time=4.142..1964.036 rows=774603 loops=1) 
          -> Hash (cost=4.340..4.340 rows=34 width=30) (actual time=0.149..0.149 rows=34 loops=1) 
           -> Seq Scan on alpha_kilo delta_yankee (cost=0.000..4.340 rows=34 width=30) (actual time=0.017..0.095 rows=34 loops=1) 
       -> Subquery Scan "*SELECT* 2" (cost=4.760..884.690 rows=104 width=366) (actual time=7.846..10.161 rows=104 loops=1) 
        -> Hash Join (cost=4.760..883.650 rows=104 width=366) (actual time=7.837..9.804 rows=104 loops=1) 
          Hash Cond: (foxtrot.xray_hotel = delta_yankee2.xray_hotel) 
          -> Nested Loop (cost=0.000..877.200 rows=104 width=340) (actual time=7.573..9.156 rows=104 loops=1) 
           -> Seq Scan on four_india foxtrot (cost=0.000..7.040 rows=104 width=216) (actual time=0.081..0.311 rows=104 loops=1) 
           -> Index Scan using three_delta on oscar alpha_victor (cost=0.000..8.350 rows=1 width=128) (actual time=0.077..0.078 rows=1 loops=104) 
             Index Cond: (quebec_seven2.lima = foxtrot.tango_three) 
          -> Hash (cost=4.340..4.340 rows=34 width=30) (actual time=0.216..0.216 rows=34 loops=1) 
           -> Seq Scan on alpha_kilo quebec_foxtrot (cost=0.000..4.340 rows=34 width=30) (actual time=0.035..0.153 rows=34 loops=1) 
       -> Subquery Scan "*SELECT* 3" (cost=4.760..457.770 rows=5778 width=141) (actual time=0.264..58.353 rows=5777 loops=1) 
        -> Hash Join (cost=4.760..399.990 rows=5778 width=141) (actual time=0.253..39.062 rows=5777 loops=1) 
          Hash Cond: (four_uniform.xray_hotel = delta_yankee3.xray_hotel) 
          -> Seq Scan on whiskey four_uniform (cost=0.000..315.780 rows=5778 width=115) (actual time=0.112..15.759 rows=5778 loops=1) 
          -> Hash (cost=4.340..4.340 rows=34 width=30) (actual time=0.117..0.117 rows=34 loops=1) 
           -> Seq Scan on alpha_kilo golf (cost=0.000..4.340 rows=34 width=30) (actual time=0.005..0.059 rows=34 loops=1) 
     -> Hash (cost=2.430..2.430 rows=10 width=4) (actual time=0.303..0.303 rows=10 loops=1) 
       -> Subquery Scan "ANY_subquery" (cost=0.000..2.430 rows=10 width=4) (actual time=0.092..0.284 rows=10 loops=1) 
        -> Limit (cost=0.000..2.330 rows=10 width=68) (actual time=0.089..0.252 rows=10 loops=1) 
          -> Index Scan using tango_seven on zulu romeo (cost=0.000..257535.070 rows=1106504 width=68) (actual time=0.087..0.227 rows=10 loops=1) 
Total runtime: 24687.975 ms 

Answer 1

Sembra che ho finalmente trovato una soluzione:

select * 
    from view1 
    where view1.id = ANY(
         (select array(select ext_id 
            from aggregate_table 
            order by somedata limit 10) 
         )::integer[] 
        ) 
    order by view1.somedata; 

Dopo elaborando @ idea di Dukeling:

Ho il sospetto dove id in (1, 2,3,4,5,6,7,8,9,10) può essere ottimizzato e dove id in (selezionare ...) non può, il motivo è che (1,2,3,4,5,6,7,8,9,10) è un'espressione costante, mentre la selezione è non.

e la localizzazione di questi nel piano di query più veloce

Recheck Cond: (id = ANY ('{1,2,3,4,5,6,7,8,9,10}'::integer[])) 
Index Cond: (id = ANY ('{1,2,3,4,5,6,7,8,9,10}'::integer[])) 

questo funziona anche più veloce della prima query in questione, circa 1.2ms, e ora utilizza

Recheck Cond: (id = ANY ($1)) 
Index Cond: (id = ANY ($1)) 

e bitmap scansioni nel piano.

Answer 2

Ho il sospetto where id in (1,2,3,4,5,6,7,8,9,10) può essere ottimizzato e where id in (select ...) non può, la ragione essendo che (1,2,3,4,5,6,7,8,9,10) è una costante e xpression, mentre lo select non lo è.

ne dite:

WITH myCTE AS 
(
    SELECT ext_id 
    FROM aggregate_table 
    ORDER BY somedata 
    LIMIT 10 
) 
SELECT * 
FROM myCTE 
LEFT JOIN table1 
    ON myCTE.ext_id = table1.id 
ORDER BY somedata