在PostgreSQL中查找表的空列

什么查询将返回所有行为NULL的表的列的名称？

测试平台：

create role stack;
create schema authorization stack;
set role stack;

create table my_table as 
select generate_series(0,9) as id,1 as val1,null::integer as val2;

create table my_table2 as 
select generate_series(0,null::integer as val2,3 as val3;

功能：

create function has_nonnulls(p_schema in text,p_table in text,p_column in text)
                returns boolean language plpgsql as $$
declare 
  b boolean;
begin
  execute 'select exists(select * from '||
          p_table||' where '||p_column||' is not null)' into b;
  return b;
end;$$;

查询：

select table_schema,table_name,column_name,has_nonnulls(table_schema,column_name)
from information_schema.columns
where table_schema='stack';

结果：

table_schema | table_name | column_name | has_nonnulls
--------------+------------+-------------+--------------
 stack        | my_table   | id          | t
 stack        | my_table   | val1        | t
 stack        | my_table   | val2        | f
 stack        | my_table2  | id          | t
 stack        | my_table2  | val1        | t
 stack        | my_table2  | val2        | f
 stack        | my_table2  | val3        | t
(7 rows)

此外,您可以通过查询目录获得一个近似答案 – 如果null_frac为零,表示没有空值,但应对“真实”数据进行双重检查：

select tablename,attname,null_frac from pg_stats where schemaname='stack';

 tablename | attname | null_frac
-----------+---------+-----------
 my_table  | id      |         0
 my_table  | val1    |         0
 my_table  | val2    |         1
 my_table2 | id      |         0
 my_table2 | val1    |         0
 my_table2 | val2    |         1
 my_table2 | val3    |         0
(7 rows)