在 PostgreSQL 中如何操作多个子查询执行顺序？

问题描述

我的 Postgresql 有问题，它的运行时间很长。这是一个示例代码。

create table t1 (
cust_id int,cust_name varchar(100),comment char(100));
insert into t1 
select i,'TESTNAME'||i,'dummyy' from generate_series(1,1000) a(i);
select * from t1;
create table t2
(id int,cust_id int,amount bigint);
insert into t2
select i,case when i < 10 then i else i+1000 end,i*10
  from generate_series(1,100) a(i);
  select * from t2; 

create table t3(
singo_id int,reg_date date,comment char(200));
insert into t3
select i,mod(i,1000),'2021-01-01'::date + mod(i,'dummyyyy'
  from generate_series(1,2000) a(i);
--I inserted 'offset' to prevent subquery collapse on purpose.
select count(*)
  from t1 a
 where exists (select 1
                 from t3 b
                where reg_date >= '2021-02-02'
                  and a.cust_id = b.cust_id
                offset 0)
   and exists (select 1
                 from t2 c
                where a.cust_id = c.cust_id
                offset 0);

--执行计划

| Aggregate (actual time=8.047..8.048 rows=1 loops=1)                                  |
|   Buffers: shared hit=1568                                                           |
|   ->  Seq Scan on t1 a (actual time=8.042..8.043 rows=0 loops=1)                     |
|         Filter: ((SubPlan 2) AND (SubPlan 1))                                        |
|         Rows Removed by Filter: 1000                                                 |
|         Buffers: shared hit=1568                                                     |
|         SubPlan 2                                                                    |
|           ->  Seq Scan on t2 c (actual time=0.005..0.005 rows=0 loops=1000)          |
|                 Filter: (a.cust_id = cust_id)                                        |
|                 Rows Removed by Filter: 99                                           |
|                 Buffers: shared hit=1000                                             |
|         SubPlan 1                                                                    |
|           ->  Seq Scan on t3 b (actual time=0.293..0.293 rows=0 loops=9)             |
|                 Filter: ((reg_date >= '2021-02-02'::date) AND (a.cust_id = cust_id)) |
|                 Rows Removed by Filter: 2000                                         |
|                 Buffers: shared hit=549

我知道我做的测试 sql 很愚蠢。我在生产系统中的真实 sql 非常复杂，子查询无法折叠。看上面的执行计划，好像是Postgresql先用t2表过滤t1表。我希望优化器做的是强制优化器首先使用 t3 表进行过滤。我怎样才能做到这一点？我将测试 sql 更改为下面的这个。但是没有用。

select count(*)
  from t1 a
 where exists (select 1
                 from t2 c
                where a.cust_id = c.cust_id
                offset 0)
   and exists (select 1
                 from t3 b
                where reg_date >= '2021-02-02'
                  and a.cust_id = b.cust_id
                offset 0);

解决方法

无法强制执行顺序 - PostgreSQL 将首先执行它认为在减少行数方面更便宜或更有效的子计划。

您可以做的是使用物化公用表表达式：

MATERIALIZED

对于 12 之前的 PostgreSQL 版本，省略 EXIF_TAG_XML_PACKET。

execution exists postgresql subquery