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Message Posted: Thu, 04 Jul 2002 @ 13:47:57 GMT


     
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Subj:   Re: Queries in Teradata
 
From:   Sanjaya Nagabhushan

1). What is Fallback and Journal in Teradata?

Software Fault Tolerance is realized in Teradata by Fallback. A fallback table is a duplicate copy of a primary table. Each row in a fallback table is stored on an AMP different from the one to which the primary row hashes. This reduces the likelihood of loss of data due to simultaneous losses of the 2 AMPs or their associated disk storage.

Journaling is another Software Fault Tolerant method. The Teradata RDBMS permits several kinds of journaling (Down AMP recovery journal, Transient Journal & Permanent Journal). The system does some journaling on its own, while a user specifies other journaling. A simple example of journal is Transient Journal - During a transaction, all before images are stored in the journal. If the transaction completes successfully, then this journal is discarded whereas if the transaction fails, the data from the journal is used to perform a rollback.

For more info refer --> Introduction to Teradata RDBMS

2). What is record mode and Indicator mode in Fast Export? what is the difference between the two?

Indicator & Record mode return data with variable-length records. Each record has a value for all the columns specified by the SELECT statement. Variable length columns are preceded by a two byte control value indicating the length of the column data. Null columns have a value that is appropriate for the column data type. The difference between the two modes is ...

Record Mode : Null values are not explicitly identified in the record parcel.

Indicator Mode : Null values are explicitly identified.

Data records returned in indicator mode have a set of bit flags that identify the columns that have a null value.

For detailed info refer --> Teradata Call-Level Interface (CLIV2)

3). How to achieve duplicate record insertion in Teradata? What abt the Multiset option for creating the table?

In ANSI mode, the default row type defined in the CREATE TABLE statement is MULTISET which means that duplicate rows are allowed when updating or inserting rows into tables.

In TERADATA mode, the default row type defined in the CREATE TABLE statement is a SET which means that duplicate rows are not allowed when updating or inserting rows into tables.

If there are uniqueness constraints on any column or set of columns in the table definition, then the table cannot have duplicate rows even if it is declared as MULTISET. Some client utilities too have restrictions with respect to MULTISET.

If neither SET nor MULTISET is explicitly specified, then assignment is made based on the session mode

ANSI - MULTISET
TERADATA - SET

Also, It is not possible to distinguish among duplicate rows in a MULTISET table. Because of this, when a WHERE condition identifies a duplicate row, then all of the duplicate rows are deleted. Unless a table is created as MULTISET (without UNIQUE constraints) to allow duplicate rows, the Teradata RDBMS always checks for duplicate rows during the update process.


Regards,

Sanjay



     
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