Difference to between COMPUTE STATISTICS and VALIDATE STRUCTURE for ANALYZE INDEX

TWO WAYS to analyze index:
ANALYZE INDEX index_name COMPUTE STATISTICS;
ANALYZE INDEX index_name VALIDATE STRUCTURE;

COMPUTE STATISTICS:

populates statistics for cost base optimizer mode

using all the target data to compute index

create histogram based statistics

It is for making better explain plan for optimizer

VALIDATE STRUCTURE:

just a diagnostic tool -- not for statistics gathering!

detect index physical/logical corruption of segment

populates some statistics in index_stats for session level

check index integrity/fragmentation

PRACTICE:
index_stats only stores a single line of data (in the same session, new value replaces the old one)

SQL> analyze index pk_emp validate structure;

Index analyzed.

SQL> select name from index_stats;

NAME
------------------------------
PK_EMP

SQL> analyze index pk_dept validate structure;

Index analyzed.

SQL> select name from index_stats;

NAME
------------------------------
PK_DEPT

SQL> disconnect;
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> conn scott/tiger
Connected.
SQL> analyze index pk_emp validate structure;

Index analyzed.

SQL> analyze index pk_dept validate structure;

Index analyzed.

SQL> select name from index_stats;

NAME
------------------------------
PK_DEPT

online/offline option Test

SQL> conn scott/tiger
Connected.
SQL> analyze index pk_emp validate structure online;

Index analyzed.

SQL> select count(1) from index_stats;

  COUNT(1)
----------
         0

SQL> analyze index pk_emp validate structure offline;

Index analyzed.

SQL> select count(1) from index_stats;

  COUNT(1)
----------
         1