考虑以下查询:
select a.id from a
where
a.id in (select b.a_id from b where b.x='x1' and b.y='y1') and
a.id in (select b.a_id from b where b.x='x2' and b.y='y2')
order by a.date desc
limit 20
哪个应该可以重写为更快的那个:
select a.id from a inner join b as b1 on (a.id=b1.a_id) inner join b as b2 on (a.id=b2.a_id)
where
b1.x='x1' and b1.y='y1' and
b2.x='x2' and b2.y='y2'
order by a.date desc
limit 20
我们不希望通过更改我们的源代码来重写我们的查询,因为它会变得非常复杂(尤其是在使用 Django 时)。
因此,我们想知道 PostgreSQL 何时将子查询折叠为连接,何时不折叠?
这是简化的数据模型:
Table "public.a"
Column | Type | Modifiers
-------------------+------------------------+-------------------------------------------------------------
id | integer | not null default nextval('a_id_seq'::regclass)
date | date |
content | character varying(256) |
Indexes:
"a_pkey" PRIMARY KEY, btree (id)
"a_id_date" btree (id, date)
Referenced by:
TABLE "b" CONSTRAINT "a_id_refs_id_6e634433343d4435353" FOREIGN KEY (a_id) REFERENCES a(id) DEFERRABLE INITIALLY DEFERRED
Table "public.b"
Column | Type | Modifiers
----------+-----------+-----------
a_id | integer | not null
x | text | not null
y | text | not null
Indexes:
"b_x_y_a_id" UNIQUE CONSTRAINT, btree (x, y, a_id)
Foreign-key constraints:
"a_id_refs_id_6e634433343d4435353" FOREIGN KEY (a_id) REFERENCES a(id) DEFERRABLE INITIALLY DEFERRED
- a 有 700 万行
- b 有 7000 万行
- b.x 的基数 = ~100
- b.y 的基数 = ~100000
- b.x 的基数,b.y = ~150000
- 设想表 c、d 和 e 具有与 b 相同的结构,并且可以额外使用以进一步减少生成的 a.ids
PostgreSQL 的版本,我们测试了查询。
x86_64-suse-linux-gnu 上的 PostgreSQL 9.2.7,由 gcc (SUSE Linux) 4.7.2 20130108 [gcc-4_7-branch revision 195012] 编译,64 位
x86_64-suse-linux-gnu 上的 PostgreSQL 9.4beta1,由 gcc (SUSE Linux) 4.7.2 20130108 [gcc-4_7-branch revision 195012] 编译,64 位
查询计划(具有空文件缓存和内存缓存):
最佳答案
我认为,您最后的评论说明了原因:这两个查询不等价,除非引入唯一约束使它们等价。
等效架构示例:
denis=# \d a
Table "public.a"
Column | Type | Modifiers
--------+---------+------------------------------------------------
id | integer | not null default nextval('a_id_seq'::regclass)
d | date | not null
Indexes:
"a_pkey" PRIMARY KEY, btree (id)
Referenced by:
TABLE "b" CONSTRAINT "b_a_id_fkey" FOREIGN KEY (a_id) REFERENCES a(id)
denis=# \d b
Table "public.b"
Column | Type | Modifiers
--------+---------+-----------
a_id | integer | not null
val | integer | not null
Foreign-key constraints:
"b_a_id_fkey" FOREIGN KEY (a_id) REFERENCES a(id)
使用该模式的等效违规数据:
denis=# select * from a order by d;
id | d
----+------------
1 | 2014-12-10
2 | 2014-12-11
3 | 2014-12-12
4 | 2014-12-13
5 | 2014-12-14
6 | 2014-12-15
(6 rows)
denis=# select * from b order by a_id, val;
a_id | val
------+-----
1 | 1
1 | 1
2 | 1
2 | 1
2 | 2
3 | 1
3 | 1
3 | 2
(8 rows)
使用两个 IN 子句的行:
denis=# select a.id, a.d from a where a.id in (select b.a_id from b where b.val = 1) and a.id in (select b.a_id from b where b.val = 2) order by d;
id | d
----+------------
2 | 2014-12-11
3 | 2014-12-12
(2 rows)
使用两个连接的行:
denis=# select a.id, a.d from a join b b1 on a.id = b1.a_id join b b2 on a.id = b2.a_id where b1.val = 1 and b2.val = 2 order by d;
id | d
----+------------
2 | 2014-12-11
2 | 2014-12-11
3 | 2014-12-12
3 | 2014-12-12
(4 rows)
不过,我看到您已经对 b (a_id, x, y) 设置了唯一约束。也许在 Postgres 性能列表中突出显示问题,以了解在您的特定情况下它没有崩溃的原因——或者至少没有生成完全相同的计划。
关于postgresql - PostgreSQL 何时将子查询合并为连接,何时不合并?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/27363263/