When the optimizer analyzes a SARG, it uses the histogram values, densities, and the number of rows in the table to estimate the number of rows that match the value specified in the SARG:
If the SARG value matches a frequency cell, the estimated number of matching rows is equal to the weight of the frequency cell multiplied by the number of rows in the table. This query includes a data value with a high number of duplicates, so it matches a frequency cell:
where authors.city = "New York"
If the weight of the frequency cell is #.015606, and the authors table has 5000 rows, the optimizer estimates that the query returns 5000 * .015606 = 78 rows.
If the SARG value falls within a range cell, the optimizer uses the range cell density to estimate the number of rows. For example, a query on a city value that falls in a range cell, with a range cell density of .000586 for the column, would estimate that 5000 * .000586 = 3 rows would be returned.
For range queries, the optimizer adds the weights of all cells spanned by the range of values. When the beginning or end of the range falls in a range cell, the optimizer uses interpolation to estimate the number of rows from that cell that are included in the range.
