postgreSQL-如何查数据库表、字段以及字段类型、注释等信息？

之前从网上也搜索了一些关于postgreSQL的系统表含义以及如何查表相关信息，但是都没有一个完整的内容，所以自己将找到的一些内容作了下整合，大家可以根据自己需要再对sql进行调整。

--1.查询对象字段、注释和字段类型
select  t.schemaname 
    ,c.relname object_name
    ,d.description object_comment
    --,c.reltuples count
    --,c.relkind object_type --r = 普通表，i = 索引，S = 序列，v = 视图， c = 复合类型，s = 特殊，t = TOAST表
    --,c.relhaspkey primary_key --f：假，t-：真
    ,a.attname column_name
    ,db.description column_comment
    ,s.attrnums distributed_column
    --,case when s.attrnums is not null then 'is distributed key' end distributed_column
    --,a.attnum column_seq
    ,case p.typname 
        when 'int8' then 'bigint'
        when 'int4' then 'integer'
        when 'int2' then 'smallint'
        when 'timestamp' then 'timestamp without time zone'
        when 'date' then 'date'
        when 'varchar' then 'character varying('||a.atttypmod - 4||')'
        when 'numeric' then  'numeric('||(a.atttypmod - 4) / 65536||','||(atttypmod - 4) % 65536||')'
    end column_type
    ,at.adsrc AS column_default
        ,CASE
         WHEN a.attnotnull THEN
          'not null'
         ELSE
          ''
       END AS column_not_null
    --,p.typname
    --,p.typlen
    --,a.atttypmod - 4 as column_length
 from pg_tables t
inner join pg_class c on t.tablename=c.relname
left join pg_description d on c.oid=d.objoid and d.objsubid=0
left join pg_description db on c.oid=db.objoid and db.objsubid>0
left join pg_attribute a on c.oid=a.attrelid and db.objsubid=a.attnum
left join ( select a.attrnums[i.i] attrnums,b.attname,a.localoid  
   from gp_distribution_policy a,  
        (select generate_series(1,10)) i(i),  --预计表分布键的个数为10个
        pg_attribute b   
  where a.attrnums[i.i] is not null   
    and a.localoid=b.attrelid   
    and a.attrnums[i.i]=b.attnum ) s on c.oid=s.localoid and a.attnum=s.attrnums
left join pg_type p on a.atttypid=p.oid
left join pg_attrdef at on c.oid=at.adrelid and a.attnum=at.adnum  
where upper(c.relname) = upper('bas_add_sub_rep') --表名
order by a.attnum;