Warum ist der "IN" -Operator bei der Verwendung mit Unterabfragen so langsam?

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

führt in 9ms aus.

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

Wird in 25000 ms ausgeführt und scheint sequentiellen Scan für die Ansicht zu verwenden (view1) anstelle der Indexsuche nach Primärschlüsseln, die von der Unterabfrage wie in der ersten Abfrage zurückgegeben wurden.

Die Unterabfrageselect ext_id from aggregate_table order by somedata limit 10 wird in 0.1ms ausgeführt

Die Verlangsamung der zweiten Abfrage wird also durch die fortlaufende Suche verursachtview1 Das ist eine Ansicht, die drei UNIONS und ungefähr drei JOINS in jeder UNION enthält. Die erste UNION enthält ungefähr 1M Zeilen, andere viel weniger. Schließt sich Tabellen mit einigen 100 KB Zeilen an. Das ist jedoch nicht so relevant, ich wollte nur das Verhalten des IN-Operators verstehen.

Was ich versuche zu erreichen, ist das Ergebnis einer Unterabfrage (ein Satz von Primärschlüsseln) zu nehmen und die Daten aus einer komplexen Sicht auszuwählen (view1) mit nur ihnen.

Ich kann auch nicht verwenden

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

weil ich die big join nicht nach sortieren willsomedata. Ich möchte nur 10 Ergebnisse aus der Ansicht nach Primärschlüsseln auswählen und dann nur diese sortieren.

Die Frage ist, warum der Operator IN schnell ist, wenn ich diese Schlüssel explizit aufführe, und so langsam, wenn ich eine schnelle Unterabfrage verwende, die genau denselben Schlüsselsatz zurückgibt.

Erläutern Sie die Analyse wie gewünscht

erste Abfrage -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

zweite Abfrage -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