15.00 - Sources for the Required Information - Teradata Database

Teradata Database Design

prodname
Teradata Database
vrm_release
15.00
category
User Guide
featnum
B035-1094-015K

Sources for the Required Information

The following table points to the sources for the data demographics requested by the Table form:

 

Information

Source

Distinct values

Varies.

This parameter determines how many unique values (including nulls) exist for the specified column.

  • If an existing electronic database contains the information, query that database for the number of distinct values in the column in one of two ways.
  • Examine current statistics for the table.
  • Execute a simple SELECT COUNT DISTINCT request over the required rows.
  • If there is no existing electronic database that contains the information, consult with your users to make a best guess estimate.
  • Maximum rows/value

    Varies.

    This parameter determines the maximum value for the specified column.

  • If there is an existing electronic database that contains the information, query that database for the value that occurs most frequently in the column in one of two ways.
  • Examine current statistics for the table.
  • Execute a simple SELECT MAX column_name request over the required rows.
  • If no existing electronic database contains the information, consult with your users to make a best guess estimate basing your estimate on known maxima, not averages.
  • Rows/null

    This parameter determines how many rows are null for the specified column.

  • If there is an existing electronic database that contains the information, query that database for the number of rows having a null in this in one of two ways.
  • Examine current statistics for the table.
  • Execute a simple SELECT COUNT column_name WHERE column_name IS NULL request over the required rows.
  • If no existing electronic database contains the information, consult with your users to make a best guess estimate basing your estimate on known maxima, not averages.
  • Typical rows/value

    This parameter determines a typical number of rows per value for the specified column.

    The typical number of rows per value for a column is not necessarily the average value, and the frequency can be so widely dispersed that a reasonably typical value cannot be determined.

    See “Maximum and Typical Column Value Frequencies” on page 160 for examples of how to determine a value for this parameter.

    Change rating

    Varied, depending on how your company operates. The following items are all likely sources for this information.

  • Users
  • Enterprise data model
  • IT policies and procedures
  • Note that the Teradata Index Wizard uses column change ratings as a parameter in determining the candidacy of columns for use as indexes.

    See the following topics for more information about how the Teradata Index Wizard uses this information:

  • SQL Request and Transaction Processing.
  • SQL Data Definition Language.
  • The change rating codes are as follows.

  • 0 means the data for this column never changes.
  • Examples include primary key columns and columns that contain historical information.

  • 1 means the data for this column rarely changes.
  • 2 - 8 are user‑determined and cover anything not covered by codes 0, 1, and 9.
  • 9 means the data for this column frequently changes.