Statistics determine the selectivity of the indexes. If an 
indexed column has unique values then the selectivity of 
that index is more, as opposed to an index with non-unique 
values. Query optimizer uses these indexes in determining 
whether to choose an index or not while executing a query. 

Some situations under which you should update statistics:
1) If there is significant change in the key values in the 
index
2) If a large amount of data in an indexed column has been 
added, changed, or removed (that is, if the distribution of 
key values has changed), or the table has been truncated 
using the TRUNCATE TABLE statement and then repopulated
3) Database is upgraded from a previous version

Statistics determine the selectivity of the indexes. If an 
indexed column has unique values then the selectivity of 
that index is more . Query optimizer uses these statistics 
in determining whether to choose an index or not while 
executing a query. 

To update statistics there is command :

update_statistics