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Don Burleson Blog 







Oracle finding missing indexes

Oracle Tips by Burleson Consulting

Finding Missing Indexes

While the super-expensive SQLAccess advisor will examine a SQL Tuning set and recommend missing indexes and materialized views, a multi-thousand dollar tool is not needed!

Remember, while not all full scans are evil, it is a sign of a missing index!  The first thing I examine is an unnecessary large-table full-table scan, a sure sign of a missing index.

I use my free plan9i.sql and plan10g.sql scripts to summarize SQL workloads and count the number of large table-full table scans, and I have loads of other scripts to aid in finding missing indexes in my Oracle script download with over 600 tuning scripts for Oracle.

 You can query v$sql _plan (using the plan9i.sql script from our code depot) revealed a huge number of large-table, full-table scans, which is one symptom of suboptimal SQL.

Sub-optimal index access to a table occurs when the optimizer cannot find an index or the most restrictive where clause in the SQL is not matched with an index.

With a missing index, the optimizer cannot find an appropriate index to access table rows, the optimizer will always invoke a full-table scan, reading every row in the table. Hence, a large-table full-table scan might indicate a sub-optimal SQL statement that can be tuned by adding an index that matches the where clause of the query.

Case study in a missing index

I had a client who reported slow SQL and looking into v$sql  revealed that the rows returned by each query were small, with this common where clause for their SQL:

WHERE customer_status = :v1 and customer_age > :v2;

This was due to a missing concatenated index on these columns, and building an index concattenated index on customer_status and customer_age resulted in a 50x performance improvement and a reduction in disk I/O by over 600 percent.

For more complete SQL tricks for finding missing indexes, see the book Advanced SQL Tuning: The Definitive Reference.


This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts:




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