ORA-01502 index or partition of such index is in unusable state

ERROR:-
-----
ORA-01502: index "owner.index_name" or partition of such index is in unusable state

SOLUTION:-
--------
REBUILD UNUSABLE INDEXES in Oracle Database

$sqlplus "/as sysdba"

select owner,index_name,table_name,status from dba_indexes where index_name='&index_name';
OWNER    INDEX_NAME    TABLE_NAME    STATUS
-----    ----------    ----------    ------
owner    index_name     table_name    INVALID

select owner,segment_name,sum(bytes)/1024/1024 "SIZE in MB" from dba_segments where segment_name='&segment_name';
OWNER    SEGMENT_NAME    SIZE IN MB   
-----    ------------    ----------   
owner    index_name           3000   

alter session set current_schema='&schema_name';
Session altered.

alter index <index_name> rebuild;
Index altered.

select owner,index_name,table_name,status from dba_indexes where index_name='&index_name';
OWNER    INDEX_NAME    TABLE_NAME    STATUS
-----    ----------    ----------    ------
owner    index_name     table_name    VALID

select owner,segment_name,sum(bytes)/1024/1024 "SIZE in MB" from dba_segments where segment_name='&segment_name';
OWNER    SEGMENT_NAME    SIZE IN MB   
-----    ------------    ----------   
owner    index_name            800

------------------------------------  OR  ------------------------------------------------------

SELECT 'ALTER INDEX '||OWNER||'.'||INDEX_NAME||' REBUILD;' FROM DBA_INDEXES WHERE STATUS = 'UNUSABLE';

This will output statements for all "unusable" indexes. Run them, so that the indexes can be "usable" again.