Difference Between Teradata Primary Index and Primary Key

7/23/2019 Difference Between Teradata Primary Index and Primary Key

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Difference between Teradata Primary Index

and Primary Key

ByRajOn October 15, 2012 1 Comment

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One must not get confused between Primary Key and Primary Index in Teradata. Primary KEY

is more of a logical thing however Primary INDEX is more of physical thing. In Teradata,

Primary INDEX is used for finding best access path for data retrieval and data insertion andPrimary KEY is used for finding each rows uniquely just like in other RDBMS. So below are

few differences between PRIMARY KEY and PRIMARY INDEX:

PRIMARY KEY PRIMARY INDEX

1 PRIMARY KEY cannot be NULL PRIMARY INDEX can be NULL

2PRIMARY KEY is notmandatory in Teradata

PRIMARY INDEX is mandatoryIn Teradata

3PRIMARY KEY does not helpin data distribution.

PRIMARY INDEX helps indatadistribution.

4PRIMARY KEY should beunique.

PRIMARY INDEX can be UNIQUE

(Unique Primary Index)

or NON UNIQUE (Primary Index).

5PRIMARY KEY is logicalimplementation.

PRIMARY INDEX is physicalimplementation.

Now we will see few scenarios to see how these two are handled in Teradata:
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PRIMARY KEY and PRIMARY INDEX in TERADATA

a) I have not defined PRIMARY INDEX or PRIMARY KEY on table what will happen

now: In this case, Teradata will check if any column is defined as UNIQUE, then it will make

that column as UNIQUE PRIMARY INDEX else first column will be created as PRIMARY

INDEX.

b) I have not defined PRIMARY INDEX however a column is defined as PRIMARY

KEY: In this case, Teradata will make the PRIMARY KEY column as UNIQUE PRIMARYINDEX of the table.

c) I have defined both PRIMARY KEY and PRIMARY INDEX on different column: In this

case, Teradata will make PRIMARY KEY column as UNIQUE SECONDARY INDEX i.e.

UNIQUE INDEX on the table.

So one must understand the importance of PRIMARY INDEX in Teradata. Generally,

PRIMARY KEY concept is taken care by UNIQUE PRIMARY INDEX in Teradataenvironment.

Secondary Index in Teradata with Example

ByRajOn December 29, 2012 2 Comments
http://usefulfreetips.com/Teradata-SQL-Tutorial/author/Raj/http://usefulfreetips.com/Teradata-SQL-Tutorial/author/Raj/http://usefulfreetips.com/Teradata-SQL-Tutorial/author/Raj/http://usefulfreetips.com/Teradata-SQL-Tutorial/secondary-index-in-teradata-with-example/#commentshttp://usefulfreetips.com/Teradata-SQL-Tutorial/secondary-index-in-teradata-with-example/#commentshttp://usefulfreetips.com/Teradata-SQL-Tutorial/secondary-index-in-teradata-with-example/#commentshttp://usefulfreetips.com/Teradata-SQL-Tutorial/wp-content/uploads/2012/10/PRIMARY-KEY-and-PRIMARY-INDEX-in-TERADATA.jpghttp://usefulfreetips.com/Teradata-SQL-Tutorial/secondary-index-in-teradata-with-example/#commentshttp://usefulfreetips.com/Teradata-SQL-Tutorial/author/Raj/


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What is Secondary Index in Teradata ? Secondary Index in Teradata provides an alternate path to

retrieve the data. It is used only for data retrieval and it has nothing to do with data storage. Fordata storage, Teradata use PRIMARY INDEX. So why do we need Secondary Index , when

Primary Index is available for Data Storage as well as Data Retrieval ? There may be some

situations when the query may not be using Primary Index column for data retrieval. In such

cases, data retrieval will be very slow. In such situations we can create Secondary Index on suchcolumns which are not part of PRIMARY Index however are used very often in JOIN conditions

or other conditions for data retrieval. Like PRIMARY INDEX, we have two types ofSECONDARY Index too:

Unique Secondary Index:

CREATE UNIQUE INDEX [COLUMN_NAME] ON TABLENAME;

Non-Unique Secondary Index:

CREATE INDEX [COLUMN_NAME] ON TABLENAME;

Creating SECONDARY INDEX may help in performance optimization however it also comes atsome cost in terms of resources. Whenever a SECONDARY index is created on table , a subtableis created on all the AMPs which hold following information:

SECONDARY INDEX VALUE || SECONDARY INDEX ROW_ID || PRIMARY INDEX

ROW_ID

So whenever we query using column defined as SECONDARY INDEX, all AMPs are asked tocheck for their sub-table if they hold that value. If yes, then AMPs retrieve the corresponding

PRIMARY INDEX row_id from their subtable. Then the AMP holding the PRIMARY INDEXrow_id is asked to retrieve respective records. Hence, Data Retrieval via Secondary Index is

always 2 AMP or more AMP operation. For NUSI [Non Unique Secondary Index] subtable is

created in the same AMP holding the PRIMARY row_id. However for USI[Unique SecondaryIndex], subtables hold the information about rows of different AMPs. Secondary Index avoids


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FULL TABLE scan. However one should collect STATS on Secondary Index columns in order

to allow Optimizer to use Secondary Index and not Full Table Scan.

Advantages of Secondary Index:

Avoids FULL Table Scan by providing alternate data retrieval path.

Enhances performances. Can be dropped and created anytime. A table may have multiple Secondary Index defined where as only one Primary Index is

permissible.

Disadvantages of Secondary Index:

Needs extra storage space for SUBTABLE. Needs extra I/O to maintain SUBTABLE. Collect STATS is required in order to avoid FULL TABLE SCAN.

To Drop Secondary Index use below command:

DROP INDEX [COLUMN_NAME] ON TABLENAME;

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Difference Between Teradata Primary Index and Primary Key