Postgresql 系统表作用解释

-- 查看所有数据库
SELECT datname, datowner, encoding FROM pg_database;

-- 查看当前数据库的所有模式
SELECT nspname, nspowner FROM pg_namespace;

-- 查看 public 模式下的所有普通表
SELECT relname AS table_name 
FROM pg_class 
WHERE relkind = 'r' 
AND relnamespace = (SELECT oid 
            FROM pg_namespace 
            WHERE nspname = 'public');

-- 查看 users 表的所有字段及类型
SELECT 
    a.attname AS column_name,
    t.typname AS data_type,
    a.attnotnull AS is_not_null
FROM pg_attribute a
JOIN pg_type t ON a.atttypid = t.oid
WHERE a.attrelid = (SELECT oid 
            FROM pg_class WHERE relname = 'users') 
    AND a.attnum > 0;

-- 查看 users 表的所有约束
SELECT conname, contype, conkey 
FROM pg_constraint 
WHERE conrelid = (SELECT oid FROM pg_class WHERE relname = 'users');

-- 查看 users 表的所有唯一索引
SELECT indexrelname AS index_name 
FROM pg_index 
JOIN pg_class t ON pg_index.indrelid = t.oid 
WHERE t.relname = 'users' AND indisunique = true;

-- 查看所有用户
SELECT rolname, rolsuper, rolcreaterole FROM pg_roles;

-- 查看 users 表的权限分配
SELECT grantee, privilege_type 
FROM information_schema.table_privileges 
WHERE table_name = 'users';

-- 查看当前所有活跃会话
SELECT usename, datname, query, state 
FROM pg_stat_activity 
WHERE state = 'active';

-- 查看表的行数和读写统计
SELECT relname, n_live_tup AS row_count, seq_scan AS full_scan_count 
FROM pg_stat_user_tables 
WHERE relname = 'users';

-- 用信息模式查询 users 表的字段
SELECT column_name, data_type, is_nullable 
FROM information_schema.columns 
WHERE table_name = 'users' AND table_schema = 'public';