让我们用测试数据创建2个表:
1)
create table test_users(
id serial primary key
);
insert into test_users
select * from generate_series(1,750000);
2)
create table test_posts(
id bigserial primary key,
user_id INT
);
CREATE index idx ON test_posts(user_id, id);
INSERT INTO test_posts (user_id) SELECT 2 FROM generate_series (1,30038);
INSERT INTO test_posts (user_id) SELECT 3 FROM generate_series (1,31036);
INSERT INTO test_posts (user_id) SELECT 4 FROM generate_series (1,24032);
INSERT INTO test_posts (user_id) SELECT 5 FROM generate_series (1,93094);
INSERT INTO test_posts (user_id) SELECT 6 FROM generate_series (1,31033);
INSERT INTO test_posts (user_id) SELECT 7 FROM generate_series (1,62063);
INSERT INTO test_posts (user_id) SELECT 724309 FROM generate_series (1,2);
INSERT INTO test_posts (user_id) SELECT 724306 FROM generate_series (1,1);
现在,让我们检查行的数量,例如从id
表中发布每个用户的最大test_posts
:
SELECT user_id, count(*) , max(id) as maxid
FROM test_posts
GROUP BY user_id
order by count(*)
所以,user_id = 724306
只有一个帖子和(在我的情况下)post maxid是:271299
。
user_id = 5
在表中有大多数帖子,在我的情况下maxid是:178200
现在我运行这个查询:
SELECT test_posts.id
FROM test_posts
JOIN test_users
ON test_posts.user_id = test_users.id
WHERE test_posts.id = 271299
explain analyze
显示:
"Merge Join (cost=8.88..9.06 rows=1 width=8) (actual time=146.561..146.563 rows=1 loops=1)"
" Merge Cond: (test_users.id = test_posts.user_id)"
" -> Index Only Scan using test_users_pkey on test_users (cost=0.42..22808.42 rows=750000 width=4) (actual time=0.012..110.091 rows=724307 loops=1)"
" Heap Fetches: 724307"
" -> Sort (cost=8.45..8.46 rows=1 width=12) (actual time=0.012..0.013 rows=1 loops=1)"
" Sort Key: test_posts.user_id"
" Sort Method: quicksort Memory: 25kB"
" -> Index Scan using test_posts_pkey on test_posts (cost=0.42..8.44 rows=1 width=12) (actual time=0.007..0.007 rows=1 loops=1)"
" Index Cond: (id = 271299)"
"Planning time: 0.214 ms"
"Execution time: 146.588 ms"
现在运行相同的查询,只需使用178200:
SELECT test_posts.id
FROM test_posts
JOIN test_users
ON test_posts.user_id = test_users.id
WHERE test_posts.id = 178200
explain analyze
来自:
"Merge Join (cost=8.88..9.06 rows=1 width=8) (actual time=0.040..0.042 rows=1 loops=1)"
" Merge Cond: (test_users.id = test_posts.user_id)"
" -> Index Only Scan using test_users_pkey on test_users (cost=0.42..22808.42 rows=750000 width=4) (actual time=0.016..0.018 rows=6 loops=1)"
" Heap Fetches: 6"
" -> Sort (cost=8.45..8.46 rows=1 width=12) (actual time=0.017..0.018 rows=1 loops=1)"
" Sort Key: test_posts.user_id"
" Sort Method: quicksort Memory: 25kB"
" -> Index Scan using test_posts_pkey on test_posts (cost=0.42..8.44 rows=1 width=12) (actual time=0.011..0.012 rows=1 loops=1)"
" Index Cond: (id = 178200)"
"Planning time: 0.249 ms"
"Execution time: 0.072 ms"
导致执行时间差异的原因是什么?
确认了!(pg-9.4.5这里)
下面的技巧将使第一个查询再次快速。
EXPLAIN (ANALYZE, BUFFERS)
SELECT p.id
FROM test_posts p
LEFT JOIN test_users u
ON p.user_id = u.id AND u.id IS NOT NULL
WHERE p.id = 271299
;
也确认在10.3上。