我有一个名为vote_pairs
的视图,它看起来像这样:
CREATE VIEW vote_pairs AS
SELECT
v1.name as name1,
v2.name as name2,
...
FROM votes AS v1
JOIN votes AS v2
ON v1.topic_id = v2.topic_id;
当使用视图进行查询时,votes
表中有大约100000行数据,这些查询需要约3秒钟才能执行。
但是,当我在名称上添加一个额外的过滤器时:
… ON v1.topic_id = v2.topic_id AND v1.name < v2.name;
运行时间增加了四倍,针对vote_pairs
的查询需要近12秒才能完成。
无论限制条件的位置如何,运行时间都是一致的……例如,如果将过滤器移动到外部查询的WHERE
子句中,查询同样会很慢:
SELECT * FROM vote_pairs WHERE name1 < name2;
发生了什么?在Postgres中进行词典排序比较慢吗?还是其他原因导致的?我该如何提高此查询的速度?
投票表:
CREATE TABLE votes (
topic_id INTEGER REFERENCES topics(id),
name VARCHAR(64),
vote VARCHAR(12)
)
CREATE INDEX votes_topic_name ON votes (topic_id, name);
CREATE INDEX votes_name ON votes (name);
EXPLAIN ANALYZE
没有名称过滤器的输出结果为:db=# CREATE OR REPLACE VIEW vote_pairs AS
db-# SELECT
db-# v1.name as name1,
db-# v2.name as name2
db-# FROM votes AS v1
db-# JOIN votes AS v2
db-# ON v1.topic_id = v2.topic_id;
CREATE VIEW
db=# EXPLAIN ANALYZE SELECT * FROM vote_pairs; QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=3956.38..71868.56 rows=5147800 width=28) (actual time=51.810..1236.673 rows=5082750 loops=1)
Hash Cond: (v1.topic_id = v2.topic_id)
-> Seq Scan on votes v1 (cost=0.00..1882.50 rows=112950 width=18) (actual time=0.019..18.358 rows=112950 loops=1)
-> Hash (cost=1882.50..1882.50 rows=112950 width=18) (actual time=50.671..50.671 rows=112950 loops=1)
-> Seq Scan on votes v2 (cost=0.00..1882.50 rows=112950 width=18) (actual time=0.004..20.306 rows=112950 loops=1)
Total runtime: 1495.963 ms
(6 rows)
使用过滤器:
db=# CREATE OR REPLACE VIEW vote_pairs AS
db-# SELECT
db-# v1.name as name1,
db-# v2.name as name2
db-# FROM votes AS v1
db-# JOIN votes AS v2
db-# ON v1.topic_id = v2.topic_id AND v1.name < v2.name;
CREATE VIEW
db=# EXPLAIN ANALYZE SELECT * FROM vote_pairs;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=3956.38..84738.06 rows=1715933 width=28) (actual time=66.688..6900.478 rows=2484900 loops=1)
Hash Cond: (v1.topic_id = v2.topic_id)
Join Filter: ((v1.name)::text < (v2.name)::text)
-> Seq Scan on votes v1 (cost=0.00..1882.50 rows=112950 width=18) (actual time=0.023..24.539 rows=112950 loops=1)
-> Hash (cost=1882.50..1882.50 rows=112950 width=18) (actual time=65.603..65.603 rows=112950 loops=1)
-> Seq Scan on votes v2 (cost=0.00..1882.50 rows=112950 width=18) (actual time=0.004..26.756 rows=112950 loops=1)
Total runtime: 7048.740 ms
(7 rows)
解释(分析,缓冲):
db=# EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM vote_pairs;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=3956.38..71345.89 rows=5152008 width=28) (actual time=56.230..1204.522 rows=5082750 loops=1)
Hash Cond: (v1.topic_id = v2.topic_id)
Buffers: shared hit=129 read=1377 written=2, temp read=988 written=974
-> Seq Scan on votes v1 (cost=0.00..1882.50 rows=112950 width=18) (actual time=0.008..20.492 rows=112950 loops=1)
Buffers: shared hit=77 read=676
-> Hash (cost=1882.50..1882.50 rows=112950 width=18) (actual time=55.742..55.742 rows=112950 loops=1)
Buckets: 2048 Batches: 8 Memory Usage: 752kB
Buffers: shared hit=52 read=701 written=2, temp written=480
-> Seq Scan on votes v2 (cost=0.00..1882.50 rows=112950 width=18) (actual time=0.004..22.954 rows=112950 loops=1)
Buffers: shared hit=52 read=701 written=2
Total runtime: 1499.302 ms
(11 rows)
db=# EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM vote_pairs WHERE name1 > name2;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=3956.38..84225.91 rows=1717336 width=28) (actual time=51.214..6422.592 rows=2484900 loops=1)
Hash Cond: (v1.topic_id = v2.topic_id)
Join Filter: ((v1.name)::text > (v2.name)::text)
Rows Removed by Join Filter: 2597850
Buffers: shared hit=32 read=1477, temp read=988 written=974
-> Seq Scan on votes v1 (cost=0.00..1882.50 rows=112950 width=18) (actual time=0.008..22.605 rows=112950 loops=1)
Buffers: shared hit=27 read=726
-> Hash (cost=1882.50..1882.50 rows=112950 width=18) (actual time=50.678..50.678 rows=112950 loops=1)
Buckets: 2048 Batches: 8 Memory Usage: 752kB
Buffers: shared hit=2 read=751, temp written=480
-> Seq Scan on votes v2 (cost=0.00..1882.50 rows=112950 width=18) (actual time=0.005..21.337 rows=112950 loops=1)
Buffers: shared hit=2 read=751
Total runtime: 6573.308 ms
(13 rows)
注意事项:
VACCUM FULL
和ANALYZE votes
命令已被执行- 8.4.11 和 9.2.3 版本的表现方式相同
EXPLAIN ANALYZE <query>
的结果吗? - Alex GaynorCREATE TABLE persons (id SERIAL PRIMARY KEY, name VARCHAR(64))
… 但这会使我的用例变得复杂(出于各种好的原因)。 - David WoleverVIEW
可以帮助你的。 - wildplasser