创建视图时重用子查询的结果

Question

在Postgresql中创建视图时，是否可以在后续子查询中重用子查询的结果？

例如，我有以下两个表：

CREATE TABLE application
(
    id INT PRIMARY KEY,
    name CHARACTER VARYING(255)
);
CREATE TABLE application_user
(
    id INT PRIMARY KEY,
    application_id INT REFERENCES application (id) ON DELETE CASCADE,
    active BOOLEAN
);

-- some sample data
INSERT INTO application (id, name) VALUES 
    (10, 'application1'), 
    (20, 'application2'), 
    (30, 'application3');
INSERT INTO application_user (id, application_id, active) VALUES 
    (1, 10, true),
    (2, 10, false),
    (3, 20, false),
    (4, 20, false),
    (5, 20, false);

我需要的视图（现在）如下所示：

CREATE VIEW application_stats AS
SELECT a.name,
    (SELECT COUNT(1) FROM application_user u 
        WHERE a.id = u.application_id) AS users,
    (SELECT COUNT(1) FROM application_user u 
        WHERE a.id = u.application_id AND u.active = true) AS active_users
    FROM application a;

这确实给了我正确的结果：

name             users    active_users
application1     2        1
application2     3        0
application3     0        0

但是，由于我要使用两次几乎相同的查询，所以效率也很低，理想情况下，我想重用第一个查询的结果。 有一种有效的方法可以做到这一点吗？

Answer 1

通常用以下方式将其表示为join / group by ：

SELECT a.name, COUNT(au.application_id) as users,
       SUM( (au.active = true)::int) as active_users
FROM application a LEFT JOIN
     application_user au
     ON a.name = au.application_id
GROUP BY a.name;

我很惊讶application没有serial主键。 但是由于您使用的是name ，所以根本不需要join ：

SELECT au.application_id, COUNT(*) as users,
       SUM( (au.active = true)::int) as active_users
FROM application_user au
GROUP BY au.application_id;

这将返回至少具有一台服务器的应用程序。

Answer 2

你应该加入这两个表，按application_id和使用count与FILTER (WHERE ...)子句只计算所需的行：

CREATE VIEW application_stats AS
SELECT a.name
       count(*) AS users,
       count(*) FILTER (WHERE u.active) AS active_users
FROM application a
   LEFT JOIN application_user u ON a.id = u.application_id
GROUP BY a.id;