SQL：如何连接具有 1+ 百万条记录的表

Question

我想使用以下查询连接两个表（“产品”表有 150 万条记录），但 15 分钟后查询仍在运行，我的电脑过热（它是 8GB RAM 的联想 v330-14ikb )，所以我停止了它。

我对索引很陌生，我尝试创建以下内容：

• 在订单上创建索引 customer_id_idx1 (customer_id)
• 在产品 (customer_id) 上创建索引 customer_id_idx2
• 在订单上创建索引 customer_id_revenues_idx(customer_id,revenues)
• 在产品上创建索引 customer_id_costs_idx(customer_id,costs)

这是查询：

SELECT a.customer_id, (SUM(a.revenues) / SUM(b.costs) :: FLOAT) AS roi
FROM orders a
JOIN products b
ON a.customer_id = b.customer_id
WHERE a.customer_id IN (
    SELECT customer_id FROM (SELECT 
        customer_id,
        COUNT(*) AS n_products
    FROM products
    GROUP BY 1
    ORDER BY 2 DESC
    LIMIT 5) x
)
GROUP BY a.customer_id  
ORDER BY roi DESC

输出应返回前 5 名客户按他们购买的产品数量计算的收入/成本比率。

我正在使用 pgadmin。有人可以解释我如何加快速度并使其编译吗？ 提前谢谢你。

Answer 1

据我所知，我认为您不需要汇总两次。

select customer_id, roi
from (
  select o.customer_id, 
         sum(o.revenues) / sum(p.costs)::float as roi, 
         count(*) as n_products
  from orders o
    join products p on o.customer_id = p.customer_id
  group by o.customer_id  
  order by n_products
  limit 5
) t
order by roi desc

或者尝试分别聚合两个表，然后将结果连接起来：

select o.customer_id, o.revenues / p.costs::numeric as roi
from (
  select customer_id, sum(revenues) as revenues
  from orders
  group by customer_id
) o
  join (
    select customer_id, sum(costs) as costs, count(*) n_products
    from products
    group by customer_id
  ) p on p.customer_id = o.customer_id
order by p.n_products desc
limit 5