Rebuild Index 加online与不加online时的plan区别

This logic is a fast full index scan, FFS, and can be seen by running an explain plan on the alter index rebuild command.
Index Fast Full Scan can only be used on an index where at least one of the columns is defined as NOT NULL .

1>

ALTER INDEX CNEDA.PK_COMPONENT_T REBUILD ;

Plan
ALTER INDEX STATEMENT  CHOOSE   
 3 INDEX BUILD UNIQUE CNEDA.PK_COMPONENT_T   
  2 SORT CREATE INDEX   
   1 INDEX FAST FULL SCAN UNIQUE CNEDA.PK_COMPONENT_T

2>

ALTER INDEX CNEDA.PK_COMPONENT_T REBUILD ONLINE;

Plan
ALTER INDEX STATEMENT  CHOOSE   
 3 INDEX BUILD UNIQUE CNEDA.PK_COMPONENT_T   
  2 SORT CREATE INDEX   
   1 TABLE ACCESS FULL CNEDA.COMPONENT_T

3>

ALTER INDEX CNEDA.IDX_T_TIMEKEY REBUILD ;

Plan
ALTER INDEX STATEMENT  CHOOSE   
 3 INDEX BUILD NON UNIQUE CNEDA.IDX_COMPONENT_T_TIMEKEY   
  2 SORT CREATE INDEX   
   1 INDEX FAST FULL SCAN NON-UNIQUE CNEDA.IDX_T_TIMEKEY

4>

ALTER INDEX CNEDA.IDX_COMPONENT_T_TIMEKEY REBUILD ONLINE;

Plan
ALTER INDEX STATEMENT  CHOOSE   
 3 INDEX BUILD NON UNIQUE CNEDA.IDX_COMPONENT_T_TIMEKEY   
  2 SORT CREATE INDEX   
   1 TABLE ACCESS FULL CNEDA.COMPONENT_T