Histogram types
Oracle uses two types of histograms for column statistics: height-balanced histograms and frequency histograms. The type of histogram is stored in the HISTOGRAM column of the *TAB_COL_STATISTICS views (USER and DBA). This column can have values of HEIGHT BALANCED, FREQUENCY, or NONE. In a height-balanced histogram, the column values are divided into bands so that each band contains approximately the same number of rows. The useful information that the histogram provides is where in the range of values the endpoints fall. In a frequency histogram, each value of the column corresponds to a single bucket of the histogram. Each bucket contains the number of occurrences of that single value. Frequency histograms are automatically created instead of height-balanced histograms when the number of distinct values is less than or equal to the number of histogram buckets specified. Frequency histograms can be viewed using the *TAB_HISTOGRAMS views.