alter session/system set skip_unusable_index = true/false ;
The point :
1, if set to true, then allow DML on table with unusable indexes(and index partitions). Or set to false to disallow DML on table.
2, DML on tables with constraints which with unusable index status will failed because data maybe violated constraint if DML allowed.
3, if DML not referencing on index column, DML will successfull dispite of this parameter .
suppose index idx_job_emp is defined on column job.
alter session set skip_unusable_index = false ;
insert into emp( empno, job, deptno) values (1,'analyzer',10) ; -- false ,because index maintenance can not be skip.
insert into emp( empno, deptno) values (1,10) ; -- successfull , because index does not indexing NULL value key.
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