分三步完成

1.在Postgre上建立一个视图

CREATE OR REPLACE VIEW public.getdbsize AS
 SELECT pg_size_pretty(pg_database_size(pg_database.datname)) AS pg_size_pretty
   FROM pg_database
  WHERE pg_database.datname = (( SELECT current_database() AS current_database));

ALTER TABLE public.getdbsize
  OWNER TO postgres;

2. 验证

SELECT pg_size_pretty  FROM public.getdbsize;

执行结果：

pg_size_pretty
7992 kB

3. 应用程序使用

StringBuilder sql = new StringBuilder("SELECT * FROM getdbsize;");
NpgsqlCommand cmd = new NpgsqlCommand(sql.ToString(),(NpgsqlConnection)conn.Connection);
NpgsqlDataReader reader = cmd.ExecuteReader();
string result = reader.GetString(0);

result经过trim，运算处理就是要的结果。

扯一点的是KB怎么还是小写k大写的B。