Non-Sampled and Sampled Statistics
When generating statistics for a database, by default all rows of the selected tables are used in the generation of statistics. These non-sampled statistics represent the most accurate statistics possible, because all data is considered.
When the base table is large, you may want to use sampled statistics. With a sufficient sampling, statistics created are almost identical to statistics created on the full table. The processing for sampled statistics is discussed in greater detail in
Sampled Optimizer Statistics.
Note: By default, sampled statistics are used for tables that have more than one million rows. To prevent sampling, use the –zns flag of optimizedb or the WITH NOSAMPLE option of the CREATE STATISTICS statement.