Webinar: Histograms: Pre-12c and Now

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To improve optimizer estimates in case of skewed data distribution , histograms can be created. Prior to 12c, based on No. of distinct values (NDV) in a column two types of histograms could be created :

if no. of buckets >= NDV, frequency histogram is created and the optimizer makes accurate estimates.

If no. of buckets < NDV, height balanced histogram is created and accuracy of optimizer estimates depends on whether a key value is (more...)