Collecting Statistics on Null - Teradata Database

Collecting Statistics on Null

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.

Collecting Statistics on the Data Dictionary

For information on collecting statistics on the Data Dictionary, see “Collecting Statistics on Data Dictionary Tables” on page 200.