When you create an index, a histogram is created on the first
column of the index. The histogram stores information about the
distribution of values in the column. Then you can use **update
statistics** to generate statistics for the minor keys of
a compound index and columns used in unindexed search clauses.

The
histogram for a column contains data in a set of steps or cells.
You can specify the number of cells can when the index is created
or when the **update statistics** command is run.
For each cell, the histogram stores a column value and a weight
for the cell.

There are two types of cells in histograms:

A

**frequency cell**represents a value that has a high proportion of duplicates in the column. The weight of a frequency cell times the number of rows in the table equals the number of rows in the table that match the value for the cell. If a column does not have highly duplicated values, there are only range cells in the histogram.**Range cells**represent a range of values. Range cell weights and the range cell density are used for estimating the number of rows to be returned when search argument values falls within a range cell.

For more information on histograms, see “Histogram displays”.