CREATE INDEX

- Create an index on one or more columns:
Syntax)

CREATE INDEX index 

ON table (column[,column]...);

ex)

CREATE INDEX 	emp_last_name_idx

ON 		employees(last_name);

• Create an index when:
  - A column contains a wide range of values
  - A column contains a large number of null values
  - One or more columns are frequently used together in a WHERE clause or a join condition
  - The table is large and most queries are expected to retrieve less than 2% to 4% of the rows in the table.
  
• Do not create an index when:
  - The columns are not often used as condition in the query.
  - The table is small or most queries are expected to retrieve more than 2% to 4% of the rows in the table
  - The table is updated frequently
  - The indexed columns are referenced as part of an expression
  

- You cannot midify indexes. ( Drop -> create)
** If you drop ta table, indexes and constraints are automatically dropped but views and sequences remain.