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Oracle indexes can go into a UNUSABLE state after maintenance operation on the table or if the index is marked as 'unusable' with an ALTER INDEX command. A direct path load against a table or partition will also leave its indexes unusable.
Queries and other operations against a table with unusable indexes will generate errors:
ORA-01502: index ‘string.string’ or partition of such index is in unusable state

The following SQL commands can be used to detect unusable indexes:

• Indexes:

SELECT owner, index_name, tablespace_name
FROM   dba_indexes
WHERE  status = 'UNUSABLE';

• Index partitions:

SELECT index_owner, index_name, partition_name, tablespace_name
FROM   dba_ind_PARTITIONS
WHERE  status = 'UNUSABLE';

• Index subpartitions:

SELECT index_owner, index_name, partition_name, subpartition_name, tablespace_name
FROM   dba_ind_SUBPARTITIONS
WHERE  status = 'UNUSABLE';

The following SQL will print out a list of alter commands that can be executed to fix unusable indexes:

• Indexes:

SELECT 'alter index '||owner||'.'||index_name||' rebuild tablespace '||tablespace_name ||';' sql_to_rebuild_index
FROM   dba_indexes
WHERE  status = 'UNUSABLE';

• Index partitions:

SELECT 'alter index '||index_owner||'.'||index_name ||' rebuild partition '||PARTITION_NAME||' TABLESPACE '||tablespace_name ||';' sql_to_rebuild_index
FROM   dba_ind_partitions
WHERE  status = 'UNUSABLE';

• Index subpartitions:

SELECT 'alter index '||index_owner||'.'||index_name ||' rebuild subpartition '||SUBPARTITION_NAME||' TABLESPACE '||tablespace_name ||';' sql_to_rebuild_index
FROM   dba_ind_subpartitions
WHERE  status = 'UNUSABLE';

or if you prefer via single PLSQL anonymous block:

set serveroutput on size unlimited

BEGIN
	FOR x IN
	(
		SELECT 'ALTER INDEX '||OWNER||'.'||INDEX_NAME||' REBUILD ONLINE PARALLEL' comm
		FROM    dba_indexes
		WHERE   status = 'UNUSABLE'
		UNION ALL
		SELECT 'ALTER INDEX '||index_owner||'.'||index_name||' REBUILD PARTITION '||partition_name||' ONLINE PARALLEL'
		FROM    dba_ind_PARTITIONS
		WHERE   status = 'UNUSABLE'
		UNION ALL
		SELECT 'ALTER INDEX '||index_owner||'.'||index_name||' REBUILD SUBPARTITION '||subpartition_name||' ONLINE PARALLEL'
		FROM    dba_ind_SUBPARTITIONS
		WHERE   status = 'UNUSABLE'
	)
	LOOP
		dbms_output.put_line(x.comm);
		EXECUTE immediate x.comm;
	END LOOP;
END;
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