如何优化SQLite3查询

Question

我正在通过一本书("使用SQLite")和Northwind数据库学习SQLite3.我编写了以下代码,按照其所在城市的客户数量订购客户,然后按名称按字母顺序排列.

SELECT ContactName, Phone, City as originalCity 
FROM Customers
ORDER BY (
      SELECT count(*) 
      FROM Customers 
      WHERE city=originalCity) 
   DESC, ContactName ASC

运行大约需要50-100毫秒.是否有一个标准的过程来优化这个查询,或者更一般地说,它的类型的查询？

Answer 1

在最常见的情况下,查询优化从读取查询优化器的执行计划开始.在SQLite中,您只需使用

EXPLAIN QUERY PLAN statement

在你的情况下,

EXPLAIN QUERY PLAN
SELECT ContactName, Phone, City as originalCity 
FROM Customers
ORDER BY (
      SELECT count(*) 
      FROM Customers 
      WHERE city=originalCity) 
   DESC, ContactName ASC

您可能还需要阅读输出

EXPLAIN statement

进入更低层次的细节.

Answer 2

一般来说（不仅是 SQLite），最好一次对所有值（城市）进行计数，并使用连接来构造查询：

SELECT ContactName, Phone, Customers.City as originalCity
FROM Customers
JOIN (SELECT city, count(*) cnt
      FROM Customers
      GROUP BY city) Customers_City_Count
ON Customers.city = Customers_City_Count.city
ORDER BY Customers_City_Count.cnt DESC, ContactName ASC

（为了防止像您的情况一样，对同一值（城市）多次计算计数）