This can happen for many reasons. Some of them are
1. You created a synonym on non-existing object by mistake. For example, you created a synonym on SCOTT.DEPT where either the SCOTT schema in not present or the DEPT table is missing.
2. You dropped an object but you did not drop the synonyms referencing the object.
3. You dropped a user, but you did not drop synonyms referencing the objects owned by that user.
4.synonym for partitioned table，在partition被add或drop之后也会变成invalid
When an object is dropped, synonyms referring to the object are not dropped. This script. generates DDL to drop synonyms whose translation is no longer valid.
USE WITH CAUTION.
rem Exludes SYS and SYSTEM users
select 'drop '||decode (s.owner,'PUBLIC','PUBLIC SYNONYM ',
from dba_synonyms s
where table_owner not in('SYSTEM','SYS')
and db_link is null
and not exists
from dba_objects o
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