PostgreSQL IN演算子とサブクエリのパフォーマンスが低い
サブクエリで「IN」演算子を使用するとなぜ遅いのですか？
_select *
from view1
where id in (1,2,3,4,5,6,7,8,9,10)
order by somedata;
_9msで実行されます。
_select *
from view1
where id in (select ext_id
from aggregate_table
order by somedata limit 10)
order by somedata;
_25000msで実行され、ビュー（_view1_）でのシーケンシャルスキャンを使用するようです。
サブクエリ_select ext_id from aggregate_table order by somedata limit 10_は0.1msで実行されます
したがって、2番目のクエリの速度低下は、3つのUNIONと各UNIONに約3つのJOINSを含むビューである_view1_の順次スキャンが原因です。最初のUNIONには約100万行が含まれ、その他ははるかに少なくなっています。約10万行のテーブルと結合します。それはそれほど関連性はありませんが、IN演算子の動作を理解したかっただけです。
私が達成しようとしているのは、サブクエリの結果（主キーのセット）を取得し、それらだけを使用して複雑なビュー（_view1_）からデータを選択することです。
私も使えない
_select v1.*
from view1 v1,
aggregate_table at
where v1.id = at.ext_id
order by at.somedata
limit 10
_大きな結合をsomedataでソートしたくないので。ビューから10個の結果を主キーで選択し、それらのみを並べ替えるだけです。
問題は、これらのキーを明示的にリストするとIN演算子が高速に実行され、まったく同じキーのセットを返す高速サブクエリを使用すると低速になるのはなぜですか？
要求に応じて分析を説明
最初のクエリ-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
_2番目のクエリ-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
_私は最終的に解決策を見つけたようです：
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;
@Dukelingのアイデアを詳しく説明した後：
（1,2,3,4,5,6,7,8,9,10）のidを最適化できる場所と（select ...）のidを最適化できない場所、その理由は（1、 2,3,4,5,6,7,8,9,10）は定数式ですが、selectはそうではありません。
より高速なクエリプランでこれらを見つける
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[]))
これは問題の最初のクエリよりもさらに速く、約1.2ms動作し、現在は
Recheck Cond: (id = ANY ($1))
Index Cond: (id = ANY ($1))
プラン内のビットマップスキャン。
where id in (1,2,3,4,5,6,7,8,9,10)は最適化でき、where id in (select ...)は最適化できないと思います。その理由は(1,2,3,4,5,6,7,8,9,10)は定数式ですが、selectは定数式ではありません。
どうですか：
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