Escribí dos funciones para responder la primera pregunta de tarea del Día 3 de Seven Databases en Seven Weeks .
Cree un procedimiento almacenado en el que pueda ingresar el título de una película o el nombre del actor que desee, y le devolverá las cinco sugerencias principales en función de las películas que el actor ha protagonizado o películas con géneros similares.
Mi primer intento es correcto pero lento. Puede tomar hasta 2000 ms para devolver un resultado.
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;
Mi segundo intento es correcto y rápido. Lo optimicé empujando el filtro hacia abajo desde el CTE hacia cada parte de la unión.
Eliminé esta línea de la consulta externa:
WHERE entity_term = query
Agregué esta línea a la primera consulta interna:
WHERE actors.name = query
Agregué esta línea a la segunda consulta interna:
WHERE movies.title = query
La segunda función tarda unos 10 ms para devolver el mismo resultado.
Nada difiere en la base de datos aparte de las definiciones de funciones.
¿Por qué PostgreSQL produce planes tan diferentes para estas dos consultas lógicamente equivalentes?
El EXPLAIN ANALYZE
plan de la primera función se ve así:
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
El EXPLAIN ANALYZE
plan de la segunda consulta se ve así:
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