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Determining which columns should be indexed

This is a warehouse database, and some of the tables are very large. I have a complex SQL query in Oracle8i that is using some of the large and small tables. I've used the explain plan to see the execution path. However, I am unable to determine what are the columns to be indexed.

Here are a couple of good rules of thumb to follow to determine which columns should be indexed:

  1. Create indexes on columns frequently used in WHERE clauses
        ex:  SELECT * FROM employees WHERE state = 'CA';
    The state column is a good candidate.
  2. Create indexes on columns that have lots of distinct values (10% or less of values are duplicate)
    ex: SSN (social security number), PHONE_NUM (phone number) = good candidates
    GENDER (male or female) or STATUS (yes/no or on/off) = bad candidates
  3. Create indexes on columns frequently used to join tables
        ex:  SELECT dept_name, emp_no, emp_name FROM emp, dept WHERE emp.dept_no
    = dept.dept_no ;
    The emp.dept_no column is a good candidate
These are three of the most common ways to determine where to create indexes but there are many more.

You can get more info by reviewing the Oracle Database Performance Guide and Reference and the Oracle Application Developers Guide Fundamentals documents located on TechNet: http://technet.oracle.com.

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