16.10 - Collecting Statistics on Null - Teradata Database

Teradata Database Administration

Product
Teradata Database
Release Number
16.10
Release Date
April 2018
Content Type
Administration
Publication ID
B035-1093-161K
Language
English (United States)

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.