我有一个vote_pairs视图,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;
并且,在投票表中有大约100k行,跨此视图的查询大约需要3秒钟才能执行.
… ON v1.topic_id = v2.topic_id AND v1.name < v2.name;
运行时间翻了四倍,在vote_pairs上完成查询需要大约12秒.
无论限制的位置如何,此运行时都是一致的…例如,如果将过滤器移动到外部查询的WHERE子句,则查询同样很慢：
SELECT * FROM vote_pairs WHERE name1 < name2;
这是怎么回事？ Postgres的词典比较速度慢吗？这是别的吗？我怎么能提高这个查询的速度？
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);
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)
并使用过滤器：
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)
EXPLAIN(ANALYZE,BUFFERS)：
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)
杂项说明：
>已经运行了VACCUM FULL和ANALYZE投票
> 8.4.11和9.2.3都以相同的方式运行
是的,文本比较有时很慢.你可能想尝试：
SELECT * FROM vote_pairs WHERE name1 > name2 collate "C";
这应该更快一些,因为它不会考虑特定于语言环境的比较规则.此外,您的解释分析结果表明您的shared_buffers可能设置得太低.
