Я написав дві функції, щоб відповісти на перше запитання домашнього завдання Дня 3 із семи баз даних за сім тижнів .
Створіть збережену процедуру, де ви можете ввести назву фільму чи ім’я актора, яке вам подобається, і воно поверне п’ятірку пропозицій на основі будь-яких фільмів, в яких знявся актор, або фільмів з подібними жанрами.
Моя перша спроба правильна, але повільна. Щоб повернути результат, це може зайняти до 2000 мс.
CREATE OR REPLACE FUNCTION suggest_movies(IN query text, IN result_limit integer DEFAULT 5)
RETURNS TABLE(movie_id integer, title text) AS
$BODY$
WITH suggestions AS (
SELECT
actors.name AS entity_term,
movies.movie_id AS suggestion_id,
movies.title AS suggestion_title,
1 AS rank
FROM actors
INNER JOIN movies_actors ON (actors.actor_id = movies_actors.actor_id)
INNER JOIN movies ON (movies.movie_id = movies_actors.movie_id)
UNION ALL
SELECT
searches.title AS entity_term,
suggestions.movie_id AS suggestion_id,
suggestions.title AS suggestion_title,
RANK() OVER (PARTITION BY searches.movie_id ORDER BY cube_distance(searches.genre, suggestions.genre)) AS rank
FROM movies AS searches
INNER JOIN movies AS suggestions ON
(searches.movie_id <> suggestions.movie_id) AND
(cube_enlarge(searches.genre, 2, 18) @> suggestions.genre)
)
SELECT suggestion_id, suggestion_title
FROM suggestions
WHERE entity_term = query
ORDER BY rank, suggestion_id
LIMIT result_limit;
$BODY$
LANGUAGE sql;
Моя друга спроба правильна і швидка. Я оптимізував це, натиснувши фільтр із CTE в кожну частину об'єднання.
Я видалив цей рядок із зовнішнього запиту:
WHERE entity_term = query
Я додав цей рядок до першого внутрішнього запиту:
WHERE actors.name = query
Я додав цей рядок до другого внутрішнього запиту:
WHERE movies.title = query
Друга функція займає приблизно 10 мс, щоб повернути той же результат.
Ніщо не відрізняється в базі даних, крім визначень функцій.
Чому PostgreSQL створює такі різні плани для цих двох логічно еквівалентних запитів?
EXPLAIN ANALYZE
План першої функції виглядає наступним чином :
Limit (cost=7774.18..7774.19 rows=5 width=44) (actual time=1738.566..1738.567 rows=5 loops=1)
CTE suggestions
-> Append (cost=332.56..7337.19 rows=19350 width=285) (actual time=7.113..1577.823 rows=383024 loops=1)
-> Subquery Scan on "*SELECT* 1" (cost=332.56..996.80 rows=11168 width=33) (actual time=7.113..22.258 rows=11168 loops=1)
-> Hash Join (cost=332.56..885.12 rows=11168 width=33) (actual time=7.110..19.850 rows=11168 loops=1)
Hash Cond: (movies_actors.movie_id = movies.movie_id)
-> Hash Join (cost=143.19..514.27 rows=11168 width=18) (actual time=4.326..11.938 rows=11168 loops=1)
Hash Cond: (movies_actors.actor_id = actors.actor_id)
-> Seq Scan on movies_actors (cost=0.00..161.68 rows=11168 width=8) (actual time=0.013..1.648 rows=11168 loops=1)
-> Hash (cost=80.86..80.86 rows=4986 width=18) (actual time=4.296..4.296 rows=4986 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 252kB
-> Seq Scan on actors (cost=0.00..80.86 rows=4986 width=18) (actual time=0.009..1.681 rows=4986 loops=1)
-> Hash (cost=153.61..153.61 rows=2861 width=19) (actual time=2.768..2.768 rows=2861 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 146kB
-> Seq Scan on movies (cost=0.00..153.61 rows=2861 width=19) (actual time=0.003..1.197 rows=2861 loops=1)
-> Subquery Scan on "*SELECT* 2" (cost=6074.48..6340.40 rows=8182 width=630) (actual time=1231.324..1528.188 rows=371856 loops=1)
-> WindowAgg (cost=6074.48..6258.58 rows=8182 width=630) (actual time=1231.324..1492.106 rows=371856 loops=1)
-> Sort (cost=6074.48..6094.94 rows=8182 width=630) (actual time=1231.307..1282.550 rows=371856 loops=1)
Sort Key: searches.movie_id, (cube_distance(searches.genre, suggestions_1.genre))
Sort Method: external sort Disk: 21584kB
-> Nested Loop (cost=0.27..3246.72 rows=8182 width=630) (actual time=0.047..909.096 rows=371856 loops=1)
-> Seq Scan on movies searches (cost=0.00..153.61 rows=2861 width=315) (actual time=0.003..0.676 rows=2861 loops=1)
-> Index Scan using movies_genres_cube on movies suggestions_1 (cost=0.27..1.05 rows=3 width=315) (actual time=0.016..0.277 rows=130 loops=2861)
Index Cond: (cube_enlarge(searches.genre, 2::double precision, 18) @> genre)
Filter: (searches.movie_id <> movie_id)
Rows Removed by Filter: 1
-> Sort (cost=436.99..437.23 rows=97 width=44) (actual time=1738.565..1738.566 rows=5 loops=1)
Sort Key: suggestions.rank, suggestions.suggestion_id
Sort Method: top-N heapsort Memory: 25kB
-> CTE Scan on suggestions (cost=0.00..435.38 rows=97 width=44) (actual time=1281.905..1738.531 rows=43 loops=1)
Filter: (entity_term = 'Die Hard'::text)
Rows Removed by Filter: 382981
Total runtime: 1746.623 ms
EXPLAIN ANALYZE
План другого запиту виглядає наступним чином :
Limit (cost=43.74..43.76 rows=5 width=44) (actual time=1.231..1.234 rows=5 loops=1)
CTE suggestions
-> Append (cost=4.86..43.58 rows=5 width=391) (actual time=1.029..1.141 rows=43 loops=1)
-> Subquery Scan on "*SELECT* 1" (cost=4.86..20.18 rows=2 width=33) (actual time=0.047..0.047 rows=0 loops=1)
-> Nested Loop (cost=4.86..20.16 rows=2 width=33) (actual time=0.047..0.047 rows=0 loops=1)
-> Nested Loop (cost=4.58..19.45 rows=2 width=18) (actual time=0.045..0.045 rows=0 loops=1)
-> Index Scan using actors_name on actors (cost=0.28..8.30 rows=1 width=18) (actual time=0.045..0.045 rows=0 loops=1)
Index Cond: (name = 'Die Hard'::text)
-> Bitmap Heap Scan on movies_actors (cost=4.30..11.13 rows=2 width=8) (never executed)
Recheck Cond: (actor_id = actors.actor_id)
-> Bitmap Index Scan on movies_actors_actor_id (cost=0.00..4.30 rows=2 width=0) (never executed)
Index Cond: (actor_id = actors.actor_id)
-> Index Scan using movies_pkey on movies (cost=0.28..0.35 rows=1 width=19) (never executed)
Index Cond: (movie_id = movies_actors.movie_id)
-> Subquery Scan on "*SELECT* 2" (cost=23.31..23.40 rows=3 width=630) (actual time=0.982..1.081 rows=43 loops=1)
-> WindowAgg (cost=23.31..23.37 rows=3 width=630) (actual time=0.982..1.064 rows=43 loops=1)
-> Sort (cost=23.31..23.31 rows=3 width=630) (actual time=0.963..0.971 rows=43 loops=1)
Sort Key: searches.movie_id, (cube_distance(searches.genre, suggestions_1.genre))
Sort Method: quicksort Memory: 28kB
-> Nested Loop (cost=4.58..23.28 rows=3 width=630) (actual time=0.808..0.916 rows=43 loops=1)
-> Index Scan using movies_title on movies searches (cost=0.28..8.30 rows=1 width=315) (actual time=0.025..0.027 rows=1 loops=1)
Index Cond: (title = 'Die Hard'::text)
-> Bitmap Heap Scan on movies suggestions_1 (cost=4.30..14.95 rows=3 width=315) (actual time=0.775..0.844 rows=43 loops=1)
Recheck Cond: (cube_enlarge(searches.genre, 2::double precision, 18) @> genre)
Filter: (searches.movie_id <> movie_id)
Rows Removed by Filter: 1
-> Bitmap Index Scan on movies_genres_cube (cost=0.00..4.29 rows=3 width=0) (actual time=0.750..0.750 rows=44 loops=1)
Index Cond: (cube_enlarge(searches.genre, 2::double precision, 18) @> genre)
-> Sort (cost=0.16..0.17 rows=5 width=44) (actual time=1.230..1.231 rows=5 loops=1)
Sort Key: suggestions.rank, suggestions.suggestion_id
Sort Method: top-N heapsort Memory: 25kB
-> CTE Scan on suggestions (cost=0.00..0.10 rows=5 width=44) (actual time=1.034..1.187 rows=43 loops=1)
Total runtime: 1.410 ms