Join/Subquery 是 fast/slow 取决于我过滤的列（不是简单的索引问题）

Question

PostgreSQL 9.3.2，由 Visual C++ build 1600 编译，64 位

每个客户可以有很多订单和推荐。现在，我想为客户创建一个包含一些统计信息的视图，其中对于每个客户，我都有一些计算列（每个客户一行）。

创建视图：

create view myview
select
    a.customer_id,
    sum(a.num) as num_orders,
    sum(b.num) as num_referrals
from
    (
    select
        customer.id as customer_id,
        count(customer.id) as num
    from
        customer
    left join
        order
    on
        order.customer_id = customer.id
    group by
        customer.id
    ) a
left join
    (
    select
        customer.id as customer_id,
        count(customer.id) as num
    from
        customer
    left join
        referral
    on
        referral.customer_id = customer.id
    group by
        customer.id
    ) b
on
    a.customer_id = b.customer_id
group by
    a.customer_id,
    b.customer_id
;

查询A（这个很快）：

select
    customer.*,
    myview.*
from
    customer
left join
    myview
on
    customer.id = myview.customer_id
where
    customer.id = 100
;

查询 B（这很慢）：

select
    customer.*,
    myview.*
from
    customer
left join
    myview
on
    customer.id = myview.customer_id
where
    customer.sex = 'M'
;

查询C（这个快）：

select
    customer.*,
    myview.*
from
    customer
left join
    myview
on
    customer.id = myview.customer_id
where
    (select id from customer where sex = 'M')
;

好的，那么为什么查询 B 在性能方面与查询 A 有如此大的不同？我想，在查询 B 中，首先是运行那些子查询而没有过滤，但我不知道如何修复它。

问题是生成查询的是我们的 ORM。所以，我无法通过执行查询 C 之类的操作来解决问题。
我希望有一种更好的方法来设计我的视图来解决问题。查询 A 和查询 B 之间 EXPLAIN 结果的主要区别是查询 B 有一些 MERGE RIGHT JOIN 操作。
有什么想法吗？

编辑：
我根据评论人的要求添加了以下信息。以下是更真实的信息（与上面简化的假设场景相反）。

create or replace view myview as
select
    a.id_worder,
    count(a.*) as num_finance_allocations,
    count(b.*) as num_task_allocations
from
    (
    select
        woi.id_worder,
        count(*) as num
    from
        worder_invoice woi
    left join
        worder_finance_task ct
    on
        ct.id_worder_finance = woi.id
    left join 
        worder_finance_task_allocation cta
    on
        cta.id_worder_finance_task = ct.id
    group by
        woi.id_worder
    ) a
left join
    (
    select
        wot.id_worder,
        count(*) as num
    from
        worder_task wot
    left join
        worder_task_allocation wota
    on
        wota.id_worder_task = wot.id
    group by
        wot.id_worder
    ) b
on
    a.id_worder = b.id_worder
group by
    a.id_worder,
    b.id_worder
;

查询 A（很快，显然我需要一个超过 10 到 post 超过 2 个链接的代表，所以这个没有解释）

select 
    *
from 
    worder a 
left outer join 
    myview b 
on 
    a.id = b.id_worder 
where 
    a.id = 100
;

查询 B（慢，EXPLAIN）

select 
    *
from 
    worder a 
left outer join 
    myview b 
on 
    a.id = b.id_worder 
where 
    a.id_customer = 200

查询C（快，EXPLAIN）

select 
    *
from 
    worder a 
left outer join 
    myview b 
on 
    a.id = b.id_worder 
where 
    a.id = (select id from worder where id_customer = 200)
;

Answer 1

尝试像这样重写您的视图：

create view myview
select
    c.customer_id,
    (
        select count(*) from order o where o.customer_id=c.customer_id
    ) num_orders,
    (
        select count(*) from referral r where r.customer_id=c.customer_id
    )
    from customer c ;

Join/Subquery 是 fast/slow 取决于我过滤的列（不是简单的索引问题）

Join/Subquery is fast/slow depending on which column I filter on (not a simple index issue)

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