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Oracle function based indexes


Oracle Tips by Burleson Consulting

 

One of the most important Oracle Silver Bullets is the function-based index.  A function-based index allows you to match any WHERE clause in an SQL statement and remove unnecessary large-table full-table scans with super-fast index range scans.

- See here how to display function based indexes.

- Here are the steps for tuning SQL with function-based indexes.

From the Oracle documentation we see a method for computing histograms on function-based indexes:

You should gather new column statistics on a table after creating a function-based index, to allow Oracle to collect column statistics equivalent information for the expression. This is done by calling the statistics-gathering procedure with the METHOD_OPT argument set to FOR ALL HIDDEN COLUMNS.

Note:  Once an FBI is created, you need to create CBO statistics, but beware that there are numerous bugs and issues when analyzing a function-based index.  See these important notes on statistics and function-based indexes.

The benefits of using a function-based index includes:

  • function-based indexes are easy to create
  • An function-based index can compute complex values without using a trigger
  • A single unction-based indexe can change hundreds of SQL statements plans 
  • A function-based-index can be built on an arithmetic expression or expression containing
  • PL/SQL, package functions, C callout or SQL built-in functions
  • The SQL optimizer can estimate selectivity more accurately if the expressions are materialized in a function-based index.

I also discuss function-based indexes in my new book "Oracle Silver Bullets", and show how single acts (like building a function-based index) can have a positive effect on your entire Oracle database.  I've got a lot of great Oracle tuning tips and an online code depot with working examples of function-based indexes:

Also, see these related notes on Function-based indexes:

 
Get the Complete
Oracle SQL Tuning Information 

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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