6

我有一个vote_pairs看起来像这样的视图：

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;

而且，表中有大约 100k 行votes，跨该视图的查询大约需要 3 秒才能执行。

但是，当我在名称上添加额外的过滤器时：

… ON v1.topic_id = v2.topic_id AND v1.name < v2.name;

运行时间翻了两番，完成查询需要将近 12 秒vote_pairs。

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WHERE外部查询的子句，则查询同样慢：

SELECT * FROM vote_pairs WHERE name1 < name2;

这是怎么回事？Postgres中的字典比较慢吗？这是别的吗？我怎样才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);

EXPLAIN ANALYZE没有名称过滤器的输出：

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)

解释（分析，缓冲区）：

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：

  • VACCUM FULL并且ANALYZE votes已经运行
  • 8.4.11 和 9.2.3 的行为方式相同
4

2 回答 2

5

是的，文本比较有时很慢。您可能想尝试：

SELECT * FROM vote_pairs WHERE name1 > name2 collate "C";

这应该会更快一些，因为它没有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。

于 2013-06-09T20:41:55.693 回答
1

我猜会增加缓慢是因为v1.name < v2.name过滤器为交叉积连接中的每一行添加了一些固定的操作集。

更有效的操作是检查v1.name <> v2.name，但随后您会得到重复的结果，例如(A,B), (B,A). 然后，我们可以将其添加回v1.name < v2.name一个WHERE子句，该子句将修剪重复项，并希望由于我们简化的过滤器而减少了行数。

尝试这个：

CREATE OR REPLACE 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 AND v1.name <> v2.name
    WHERE v1.name < v2.name;

（编辑：似乎COLLATE "C"是要走的路，但我会留下这个答案，因为这是减少行暴露于慢速操作的好技巧。）

于 2013-06-09T20:36:12.303 回答