What is the need of rebuilding of an index?

          ANALYZE INDEX  INDEX_NAME VALIDATE STRUCTURE;

• Using above statement we can rebuild a Index.
• Each command creates a single row in the V$INDEX_STATS view. This row is overwritten by the next ANALYZE INDEX command, so copy the contents of the view into a local table after each ANALYZE. 
• The 'badness' of the index can then be identified DEL_LF_ROWS/LF_ROWS.

What is Row Level Lock in Oracle?

• Oracle automatically acquires locks which are row-level locks. A transaction can held no of row locks and oracle does not work on it.
• Row locking provides the lowest level of locking. It provides the best possible transaction concurrency.Readers of data do not wait for writers of the same data rows.  
• A modified row is always locked exclusively so that other users cannot modify the row until the transaction holding the lock is committed or rolled back.
• If a transaction obtains a row lock for a row, the transaction also acquires a table lock for the corresponding table. The table lock prevents conflicting DDL operations that would override data changes in a current transaction.
• A Transaction gets an exclusive DML row level lock for each row modified by any of the following statements: INSERT, UPDATE, DELETE, and SELECT with the FOR UPDATE clause.