When you collect statistics, Teradata Database records three different null counts: NumNulls, PNullUniqueValueCount, and AllNulls.
- NumNulls is the number of rows that have a null in at least one field. This includes rows with all fields null.
- PNullUniqueValueCount is the number of unique values in rows with partial nulls.
- All-Nulls is the number of rows that have all fields null. For example, in the following table, NumNulls count is 4, All-Nulls count is 2, and PNullUniqueValueCount is 2:
x y ------ ------- 1 a ? b ? ? 4 ? ? ?
Use the SHOW STATISTICS statement to report the number of nulls after collecting statistics. By considering NumNulls, All-Nulls, and PNullUniqueValueCount counts, Teradata can better estimate the number of unique values in the presence of nulls.