Is Oracle Picking The Wrong Index?

At the 15th annual Hotsos Symposium I was fortunate enough to do a presentation on the second day. My presentation focused on using SQL Monitoring reports to tune statements. I went over numerous examples to show useful information provided by the reports.

One of the examples is common enough that I wanted to include in a blog post. Statements picking the wrong index is very easy to see in the report. A couple of screen shots will clearly show that time is being spent due to a poor index choice. The columns of note are drastic changes in the Actual Rows or a high percentage of Activity.

In the following report snippet you can see from the Activity percentage column that most time is spent on the step filtering out index results in the table access. Using the index access there are 23 million rows returned which are then filtered at the table level to only leave 8 rows. This is a lot of work performed on the database to return a small number of rows.

Now the problem is clear but the solution may be more difficult. The resolution will depend on the actual problem.  Either a better index exists that Oracle is not selecting due to some issue or a better index needs to be created. Some examples for resolving this problem include:
1. Update object level statistics
If there is a better index to be chosen then this step may be enough to help the optimizer make the best choice.

2. Create extended column statistics and histogram
If there is a better index that is not being chosen, you may need to create extended column statistics with a histogram. Oracle may be making a poor estimate on rows returned if there are two or more related columns. The extended column statistics and histogram will give the optimizer the information it needs to build the best plan.

3. Create a better index
If there is no existing index that returns a smaller number of rows, then you may need to create an index on the appropriate columns. You can look at the filter predicates on the table to determine which columns to index. In the SQL Monitoring report you can click on the green filter icon on the plan line to see which columns should be added to the index. To limit the rows indexed, the columns can be investigated to determine the minimum number of rows need to limit the rows returned.