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Histograms are special statistics that exist for a limited number of columns and are created by the database administrator. Column histograms should be created only when there are highly- skewed values in a column, such as is the case of an order details table with an Order Status column where the number of closed orders for a business operating for several years is far greater than the number of open orders. The Order Status column therefore meets the criteria of a useful target for a histogram because the data is highly skewed. Using histograms the optimizer determines that a full-scan is recommended when searching for closed orders, but an index scan is more useful when searching for open orders.
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Column Statistics and Histograms
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tab example
SQL Query Tuner looks at the columns that have histograms and using statistics tries to determine whether the column is a good or bad candidate for a histogram and presents this information on the Column Statistics and Histograms tab.
The row shading indicates the following:
- Green: Good histogram candidate
- Red: Bad histogram candidate
- No shading: Not determined to be a good or bad histogram candidate
Median Value Deviation
For columns that have histograms, the median value deviation is presented. Understanding the median value deviation can help you determine whether an index scan or a full-table scan would be more efficient.
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If you are gathering statistics for a column for which the statistics were missing or out- of-date, then once the statistics collection is complete, you should return to the Overview tab and rerun the cases, because the characteristics of the column may have changed, so the hints to improve performance would also change. |