15.00 - Determining If Bit Mapping Is Being Used - Teradata Database

Teradata Database Design

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

Determining If Bit Mapping Is Being Used

To determine whether bit mapping is being used for your NUSIs, use the EXPLAIN request modifier and examine the reports it produces for your queries.

Example  

The following example illustrates the bit mapping process.

The table queried contains the following set of rows.

 

The query ANDs three equality conditions on three NUSI columns in the WHERE clause.

     SELECT *
     FROM Base_Table
     WHERE column_1 = ‘A’
     AND   column_2 = ‘B’
     AND   column_3 = ‘C’;

The Optimizer evaluates the query and determines that bit mapping over the three columns is the least costly access plan.

The truth table looks like this.

 

NUSI Column     Number

 

      Value Required To        Satisfy Condition

 

                              Base Table RowID List

12,07

22,03

48,12

63,15

1

A

1

1

1

0

2

B

1

0

1

1

3

C

1

0

0

0

ANDed Sum:

1

0

0

0

Of the four rows examined in the example, only the row identified by RowID 12,07 qualifies and is returned to the requesting application.

Related Topics

Also see the topic “EXPLAIN Request Modifier” in SQL Data Manipulation Language, the chapter “Interpreting the Output of the EXPLAIN Request Modifier” in SQL Request and Transaction Processing, and Teradata Visual Explain User Guide.