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Recommended index not used

One user writes, "Using SQL performance monitor of Operation Navigator, an index was recommended. I created the recommended index, but it looks like the system doesn't use it. I find it strange that the index is recommended and then not used. Is this normal?" Db2 expert Kent Milligan says yes and explains the reason the index is not always used.

Using SQL performance monitor of Operation Navigator, an index was recommended. I created the recommended index, but it looks like the system doesn't use it. I find it strange that the index is recommended and then not used. Is this normal?
Even though the query optimizer advises creating an index, it doesn't always choose to use it when creating its access plan. There are some reasons for this:

The query optimizer does not always advise the perfect index. The reason for this is that currently, only local selection is used when determining what index to advise -- it does not yet consider join, order by, or group by columns. As a result, after you create that advised index, it may not choose to use it, as it is still not the perfect index. You may need to manually intervene -- examine the query and consider the join, group by, and order by columns to determine what the perfect index is and create that index.

Alternatively, it may be recommending that index so that it can extract stats from the index to further evaluate the optimal access method. For example, once the index has been created, it may be using statistics from the index to determine that query will return many rows, thus a table scan is the least expensive access method.

To better understand the concept of a perfect index and how indexes help the query optimizer with statistics, you can read the Indexing Strategy white paper online here.

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This was last published in August 2005

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