Gather database statics

There are several ways to gather statics in Oracle. It all depends on how wide you want to do your collection.
To gather statics for the whole database:

SQL> EXEC DBMS_STATS.gather_database_stats;
To gather statics for a single schema:
SQL> EXEC DBMS_STATS.gather_schema_stats('SCOTT');
To gather statics for a single table:SQ
L> EXEC DBMS_STATS.gather_table_stats('SCOTT', 'EMPLOYEES');
To gather statics for a single index:
SQL> EXEC DBMS_STATS.gather_index_stats('SCOTT', 'EMPLOYEES_PK');

For more information see:
http://www.oracle-base.com/articles/8i/CostBasedOptimizerAndDatabaseStatistics.php