sql in Oracle to check whether the index needs to be reconstructed

 
SELECT 
height, /*Height of the B-Tree*/ 
blocks, /* Blocks in the index segment */ 
name, /*index name */ 
lf_rows, /* number of leaf rows in the index */ 
lf_blks, /* number of leaf blocks in the index */ 
del_lf_rows, /* number of deleted leaf rows in the index */ 
rows_per_key /* average number of rows per distinct key */ 
blk_gets_per_access /* consistent mode block reads (gets) */ 
FROM INDEX_STATS 
WHERE NAME='INDEX_NAME'; 

 
ANALYZE index INDEX_NAME VALIDATE STRUCTURE 

HEIGHT:
This column refers to the height of the B-tree index, and it's usually at the 1, 2, or 3 level.
If large inserts push the index height beyond a level of 4, it's time to rebuild, which flattens the B-tree.

DEL_LF_ROWS:
This is the number of leaf nodes deleted due to the deletion of rows.
Oracle doesn't rebuild indexes automatically and, consequently, too many deleted leaf rows can lead to an unbalanced B-tree.

BLK_GETS_PER_ACCESS:
You can look at the BLK_GETS_PER_ACCESS column to see how much logical I/O it takes to retrieve data from the index. If this row shows a double-digit number, you should probably start rebuilding the index.