Gather stats Query for Database,Schema and Table.

Gather Stats for a database :
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    exec dbms_stats.gather_database_stats(estimate_percent => 15, cascade => true);

Gather stats for a schema:
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    exec dbms_stats.gather_schema_stats('newuser', estimate_percent => 15, cascade => true);

Gather stats for a table:
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    exec dbms_stats.gather_table_stats('newtab', 'newtab2', estimate_percent => 15, cascade => true);