1.查询数据库占用磁盘空间大小：

select datname, pg_size_pretty (pg_database_size(datname)) AS size from pg_database;

2.查看指定数据库 每个表的磁盘空间 倒叙排列：

SELECT
table_schema || '.' || table_name
AS table_full_name, pg_size_pretty(pg_total_relation_size('"' ||table_schema || '"."' || table_name || '"')) AS size
FROM
information_schema.tables
ORDER BY
pg_total_relation_size('"' || table_schema || '"."' || table_name || '"') desc

3.postgresql将一张表的数据copy到另一张表中

-- 创建目标表（若已存在则跳过此步骤）

CREATE TABLE target_table (

id SERIAL PRIMARY KEY,

name VARCHAR(50),

age INTEGER

);

-- 从源表复制数据到目标表

INSERT INTO target_table (name, age)

SELECT name, age FROM source_table;

vacuum 建议日常使用

vacuum full 建议大量update、delete后使用

vacuum full 会清理整个数据库

vacuum full tablename 会清理指定表的空间

可阅读参考：

http://www.postgres.cn/docs/10/routine-vacuuming.html