PostgreSQL-forespørgsel kører hurtigere med indeksscanning, men motoren vælger hash join

Mit gæt er, at du bruger standarden random_page_cost = 4 , hvilket er alt for højt, hvilket gør indeksscanning for dyr.

Jeg forsøger at rekonstruere de 2 tabeller med dette script:

CREATE TABLE replays_game (
    id integer NOT NULL,
    PRIMARY KEY (id)
);

CREATE TABLE replays_playeringame (
    player_id integer NOT NULL,
    game_id integer NOT NULL,
    PRIMARY KEY (player_id, game_id),
    CONSTRAINT replays_playeringame_game_fkey
        FOREIGN KEY (game_id) REFERENCES replays_game (id)
);

CREATE INDEX ix_replays_playeringame_game_id
    ON replays_playeringame (game_id);

-- 150k games
INSERT INTO replays_game
SELECT generate_series(1, 150000);

-- ~150k players, ~2 games each
INSERT INTO replays_playeringame
select trunc(random() * 149999 + 1), generate_series(1, 150000);

INSERT INTO replays_playeringame
SELECT *
FROM
    (
        SELECT
            trunc(random() * 149999 + 1) as player_id,
            generate_series(1, 150000) as game_id
    ) AS t
WHERE
    NOT EXISTS (
        SELECT 1
        FROM replays_playeringame
        WHERE
            t.player_id = replays_playeringame.player_id
            AND t.game_id = replays_playeringame.game_id
    )
;

-- the heavy player with 3000 games
INSERT INTO replays_playeringame
select 999999, generate_series(1, 3000);

Med standardværdien 4:

game=# set random_page_cost = 4;
SET
game=# explain analyse SELECT "replays_game".*
FROM "replays_game"
INNER JOIN "replays_playeringame" ON "replays_game"."id" = "replays_playeringame"."game_id"
WHERE "replays_playeringame"."player_id" = 999999;
                                                                     QUERY PLAN                                                                      
-----------------------------------------------------------------------------------------------------------------------------------------------------
 Hash Join  (cost=1483.54..4802.54 rows=3000 width=4) (actual time=3.640..110.212 rows=3000 loops=1)
   Hash Cond: (replays_game.id = replays_playeringame.game_id)
   ->  Seq Scan on replays_game  (cost=0.00..2164.00 rows=150000 width=4) (actual time=0.012..34.261 rows=150000 loops=1)
   ->  Hash  (cost=1446.04..1446.04 rows=3000 width=4) (actual time=3.598..3.598 rows=3000 loops=1)
         Buckets: 1024  Batches: 1  Memory Usage: 106kB
         ->  Bitmap Heap Scan on replays_playeringame  (cost=67.54..1446.04 rows=3000 width=4) (actual time=0.586..2.041 rows=3000 loops=1)
               Recheck Cond: (player_id = 999999)
               ->  Bitmap Index Scan on replays_playeringame_pkey  (cost=0.00..66.79 rows=3000 width=0) (actual time=0.560..0.560 rows=3000 loops=1)
                     Index Cond: (player_id = 999999)
 Total runtime: 110.621 ms

Efter at have sænket den til 2:

game=# set random_page_cost = 2;
SET
game=# explain analyse SELECT "replays_game".*
FROM "replays_game"
INNER JOIN "replays_playeringame" ON "replays_game"."id" = "replays_playeringame"."game_id"
WHERE "replays_playeringame"."player_id" = 999999;
                                                                  QUERY PLAN                                                                   
-----------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=45.52..4444.86 rows=3000 width=4) (actual time=0.418..27.741 rows=3000 loops=1)
   ->  Bitmap Heap Scan on replays_playeringame  (cost=45.52..1424.02 rows=3000 width=4) (actual time=0.406..1.502 rows=3000 loops=1)
         Recheck Cond: (player_id = 999999)
         ->  Bitmap Index Scan on replays_playeringame_pkey  (cost=0.00..44.77 rows=3000 width=0) (actual time=0.388..0.388 rows=3000 loops=1)
               Index Cond: (player_id = 999999)
   ->  Index Scan using replays_game_pkey on replays_game  (cost=0.00..0.99 rows=1 width=4) (actual time=0.006..0.006 rows=1 loops=3000)
         Index Cond: (id = replays_playeringame.game_id)
 Total runtime: 28.542 ms
(8 rows)

Hvis jeg bruger SSD, ville jeg sænke den yderligere til 1.1.

Med hensyn til dit sidste spørgsmål, så synes jeg virkelig, du skal holde dig til postgresql. Jeg har erfaring med postgresql og mssql, og jeg er nødt til at lægge en tredobbelt indsats i det senere, for at det fungerer halvt så godt som det tidligere.