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Fast Full Index Scan

Oracle Tips by Burleson Consulting

Index full scans are sometimes called fast full-index scans, which were introduced in Oracle 7.3. There are some SQL queries that can be resolved by reading the index without touching the table data. For example, the following query does not need to access the table rows, and the index alone can satisfy the query.

select distinct
   color,
   count(*)
from
   automobiles
group by
   color;

While some queries have always been able to return the desired result without touching the table data, Oracle enhanced the fast full-index scan to make it behave similar to a full-table scan. Just as Oracle has implemented the initialization parameter db_file_multiblock_read_count for full-table scans, Oracle allows this parameter to take effect when retrieving rows for a fast full-index scan. Since the whole index is accessed, Oracle allows multiblock reads. Also, a fast full-index scan is capable of using Oracle parallel query to further speed up the response time.

There is a huge benefit to not reading the table rows, but there are some requirements for Oracle to invoke the fast full-index scan.

  • All of the columns required must be specified in the index. That is, all columns in the select and where clauses must exist in the index.

  • The query returns more than 10 percent of the rows within the index. This 10 percent figure depends on the degree of multiblock reads and the degree of parallelism.

  • You are counting the number of rows in a table that meet a specific criterion. The fast full-index scan is almost always used for count(*) operations.

The cost-based optimizer will make the decision about whether to invoke the fast full-index scan in accordance with the table and index statistics.  You can also force a fast full-index scan by specifying the index_ffs hint:

select distinct /*+ index_ffs(c,pk_auto) /*
   color,
   count(*)
from
   automobiles
group by
   color;  

It is not always intuitive whether a fast full-index scan is the fastest way to service a query, because of all of the variables involved. Hence, most expert SQL tuners will time any query that meets the fast full-index scan criteria and see if the response time improves. Again, this is almost always fastest with count(*) operations. For counting operations, the fast full-index scans with parallel_index is clearly the fastest access method.

Next, let’s examine a very important SQL operation, the SQL join. It is through the join that Oracle navigates through the database and presents information from different tables based on a common column value.


This is an excerpt from "Oracle High-Performance SQL Tuning" by Donald K. Burleson, published by Oracle Press.


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