postgres=# DROP USER joe  CASCADE;
DROP ROLE
当在多个用户多个数据库多个schema赋予权限的场景下，drop某个用户，由于该用户拥有其他用户赋予的权限，因此直接删除会产生如下报错。
postgres=#  drop user test1 cascade;
ERROR:  role "test1" cannot be dropped because some objects depend on it
在这种场景只能通过查看系统表找到该用户被其他用户赋予的权限，并且登录该用户手动删除其赋予的权限，才能执行drop user，下面通过一个例子来详细讲解如何处理这种场景。
登录系统库，查看该用户的OID
postgres=#  select oid from pg_roles where rolname='test1';
  oid
-------
 16386
(1 row)
通过查看pg_shdepend 该视图来获取该用户被赋予的权限，可以看到该用户存在被其他用户赋予的两个权限，先处理第一个。
postgres=# select * from pg_shdepend where refobjid='16386';
 dbid  | classid | objid | objsubid | refclassid | refobjid | deptype | objfile
-------+---------+-------+----------+------------+----------+---------+---------
 16394 |     826 | 16400 |        0 |       1260 |    16386 | a       |
 16394 |     2615 | 16399 |        0 |       1260 |    16386 | a       |

查看pg_database 视图获取所在数据库，根据结果可知在test数据库。
postgres=#   select * from pg_database where oid='16394'
 datname | datdba | encoding | datcollate  |  datctype   | datistemplate | datallowconn | datconnlimit | datlastsysoid | datfrozenxid | dattablespace | datcompatibility |                      datacl
             | datfrozenxid64
---------+--------+----------+-------------+-------------+---------------+--------------+--------------+---------------+--------------+---------------+------------------+--------------------------------------
-------------+----------------
 test    |     10 |        7 | en_US.UTF-8 | en_US.UTF-8 | f             | t            |           -1 |         16268 | 1985         |          1663 | MYSQL            | {=Tc/wangwei,wangwei=CTc/wangwei,admi
n=c/wangwei} |           1985
(1 row)
登录到test数据库
[wangwei@euler_phy_194 postgres]$ gsql -p 3730 -d test
gsql (( GaussDB  Kernel V500R002C00 build 43ff9cd7) compiled at 2021-04-17 14:30:45 commit 0 last mr 330 debug)
Non-SSL connection (SSL connection is recommended when requiring high-security)
Type "help" for help.
通过pg_class 查看权限的类型，where条件的oid即是之前pg_shdepend 视图中查到的classid。
test=# select * from pg_class where oid = 826;
    relname     | relnamespace | reltype | reloftype | relowner | relam | relfilenode | reltablespace | relpages | reltuples | relallvisible | reltoastrelid | reltoastidxid | reldeltarelid | reldeltaidx | rel
cudescrelid | relcudescidx | relhasindex | relisshared | relpersistence | relkind | relnatts | relchecks | relhasoids | relhaspkey | relhasrules | relhastriggers | relhassubclass | relcmprs | relhasclusterkey
 | relrowmovement | parttype | relfrozenxid |    relacl    | reloptions | relreplident | relfrozenxid64 | relbucket | relbucketkey
----------------+--------------+---------+-----------+----------+-------+-------------+---------------+----------+-----------+---------------+---------------+---------------+---------------+-------------+----
------------+--------------+-------------+-------------+----------------+---------+----------+-----------+------------+------------+-------------+----------------+----------------+----------+-----------------
-+----------------+----------+--------------+--------------+------------+--------------+----------------+-----------+--------------
 pg_default_acl |           11 |   11810 |         0 |       10 |     0 |       16063 |             0 |        0 |         0 |             0 |             0 |             0 |             0 |           0 |
          0 |            0 | t           | f           | p              | r       |        4 |         0 | t          | f          | f           | f              | f              | 0        | f
 | f              | n        | 0            | {=r/wangwei} |            | n            |           1985 |           |
(1 row)
可以看到该用户被其他用户授予了表或者视图的ACL默认权限，查看pg_default_acl视图，oid即是之前pg_shdepend 中的查到的objid。
test=#  select * from pg_default_acl oid = 16400;;
 defaclrole | defaclnamespace | defaclobjtype |               defaclacl               |  oid
------------+-----------------+---------------+---------------------------------------+-------
      16395 |           16399 | r             | {wangwei=arwd/admin,test1=arwd/admin} | 16400
(1 row)
根据该视图的defaclacl字段可以看到，test1用户被admin用户赋予了默认权限，下一步我们需要找到具体授予权限的对象(哪个表或者视图）。
通过pg_namespace 查看对象的名字，其中oid即是pg_default_acl 中查到的defaclnamespace，通过视图可以看出test1用户被admin用户授予了schema test_schema_1的权限。 
test=# select * from pg_namespace where oid = 16399;
    nspname    | nspowner | nsptimeline |                nspacl                 | in_redistribution | nspblockchain
---------------+----------+-------------+---------------------------------------+-------------------+---------------
 test_schema_1 |       10 |           0 | {wangwei=UC/wangwei,admin=UC/wangwei} | n                 | f
(1 row)
用admin登录test数据库，执行ALTER DEFAULT PRIVILEGES IN SCHEMA test_schema_1 revoke all privileges ON TABLES from test1;收回admin用户向test1赋予的scheme 默认权限。
test=#   ALTER DEFAULT PRIVILEGES IN SCHEMA test_schema_1 revoke all privileges ON TABLES from test1;
ALTER DEFAULT PRIVILEGES

下面处理第二个权限，登录系统库，查看pg_database 视图获取所在数据库，根据结果可知在test数据库。
postgres=#   select * from pg_database where oid='16394'
 datname | datdba | encoding | datcollate  |  datctype   | datistemplate | datallowconn | datconnlimit | datlastsysoid | datfrozenxid | dattablespace | datcompatibility |                      datacl
             | datfrozenxid64
---------+--------+----------+-------------+-------------+---------------+--------------+--------------+---------------+--------------+---------------+------------------+--------------------------------------
-------------+----------------
 test    |     10 |        7 | en_US.UTF-8 | en_US.UTF-8 | f             | t            |           -1 |         16268 | 1985         |          1663 | MYSQL            | {=Tc/wangwei,wangwei=CTc/wangwei,admi
n=c/wangwei} |           1985
(1 row)
登录到test数据库。
[wangwei@euler_phy_194 postgres]$ gsql -p 3730 -d test

查看视图pg_class ，其中oid即是pg_shdepend 表中的第二行的classid。
postgres=#  select * from pg_class where oid='2615';
   relname    | relnamespace | reltype | reloftype | relowner | relam | relfilenode | reltablespace | relpages | reltuples | relallvisible | reltoastrelid | reltoastidxid | reldeltarelid | reldeltaidx | relcu
descrelid | relcudescidx | relhasindex | relisshared | relpersistence | relkind | relnatts | relchecks | relhasoids | relhaspkey | relhasrules | relhastriggers | relhassubclass | relcmprs | relhasclusterkey |
 relrowmovement | parttype | relfrozenxid |    relacl    | reloptions | relreplident | relfrozenxid64 | relbucket | relbucketkey
--------------+--------------+---------+-----------+----------+-------+-------------+---------------+----------+-----------+---------------+---------------+---------------+---------------+-------------+------
----------+--------------+-------------+-------------+----------------+---------+----------+-----------+------------+------------+-------------+----------------+----------------+----------+------------------+
----------------+----------+--------------+--------------+------------+--------------+----------------+-----------+--------------
 pg_namespace |           11 |   11787 |         0 |       10 |     0 |       15947 |             0 |        1 |        20 |             1 |             0 |             0 |             0 |           0 |
        0 |            0 | t           | f           | p              | r       |        6 |         0 | t          | f          | f           | f              | f              | 0        | f                |
 f              | n        | 0            | {=r/wangwei} |            | n            |           1985 |           |
(1 row)

根据结果可以看出是pg_namespace类型，查看pg_namespace系统视图，oid即是pg_shdepend 第二行objid。
test=# select * from pg_namespace where oid='16399';
    nspname    | nspowner | nsptimeline |                        nspacl                         | in_redistribution | nspblockchain
---------------+----------+-------------+-------------------------------------------------------+-------------------+---------------
 test_schema_1 |       10 |           0 | {wangwei=UC/wangwei,admin=UC/wangwei,test1=U/wangwei} | n                 | f
(1 row)

通过该视图的nspacl可以看出test1用户的test_schema_1 视图被权限被wangwei用户赋权，因此通过wangwei用户登录test库，执行revoke all on schema test_schema_1 from test1,回收赋予test1用户的权限。
test=# revoke all on schema test_schema_1 from test1;
REVOKE

至此test1用户的被其他用户赋予的权限已经全部清除了，接下来只要执行drop user命令就可以了。
postgres=# drop user test1 cascade;
DROP ROLE