Teradata-Secondary Index

Secondary Indexes provide an alternate path to the data, and should be used on queries that run thousands of times.

Teradata Secondary Index

Teradata runs extremely well without secondary indexes, but since secondary indexes use up space and overhead, they should only be used on "KNOWN QUERIES" or queries that are run over and over again. Once you know the data warehouse, environment you can create secondary indexes to enhance its performance.

Whenever a secondary index is created, Teradata creates a secondary index subtable on each AMP. All secondary index subtables contain:

  1. Secondary Index Value 
  2. Secondary Index Row ID 
  3. Primary Index Row ID

Secondary indexes are two types

A UNIQUE Secondary Index (USI) will improve data retrieval and can also be used to enforce uniqueness on a primary key. Typically, only two AMPs are used on a Unique Secondary Index (USI) access.

A Non-Unique Secondary Index (NUSI) is AMP local and is an All AMP operation, but not a full table scan.

Comments

Post a Comment

Popular posts from this blog

SQL for Quantile Function in Teradata

Image
A Quantile is used to divide rows into a number of categories or grouping of roughly the same number of rows in each group. Quantile Function The percentile is the QUANTILE most commonly used in business. This means that the request is based on a value of 100 for the number of partitions SELECT Product_ID1, Sales_Date1, Daily_Sales1 ,QUANTILE(100, Daily_Sales1 ) AS "Quantile1" FROM Sales_Table WHERE Product_ID1 < 3000 AND Sales_Date1 > 1000930 ; The calculation is percentile for every row in the Sales table on Daily sales.
Read more

3 Uses of SAMPLE function in Teradata

Image
Usage-1 It returns a rows randomly from Teradata database. Syntax for SAMPLE function: SAMPLE [WITH REPLACEMENT]    [RANDOMIZED ALLOCATION]                  [WHEN <condition>  THEN]                  {<number-of-rows> | <percentage>}    […,<number-of-rows> | <percentage>]                  [ELSE {<number-of-rows> |    <percentage }   END] Usage 2 It allows the user to get absolute number of rows or percentage of rows. Usage 3 It also allows the rows to get from multiple samples How to get random rows from a table: SELECT * from sample_table SAMPLE 5; How to get certain percentage of rows: SELECT * from sample_table SAMPLE .30; How to get multiple SAMPLE data from a single tables with out duplicates: SELECT * from sample_table SAMPLE .20, SAMPLE .30 ORDER BY 1,2; SQL query to get 3 samples with duplicates: So 'WITH REPLACEMENT" you will get duplicate rows: SELECT ST_
Read more

All You Need NULL Value Functions

Image
The below are the Null value functions in Tearadata. One is "NVL" and another one is "NVL2". These two functions are part of Teradata SQL . NVL -The NVL function replace null value with a string or numeric value. This is the functionality is same as Teradata function NULLIF. It requires two arguments. If the value is not null, it will not do anything. The best SQL Null functions  NVL function:  SELECT NVL (NULL,'USA'), NVL('SPAIN','USA'); ----------- USA SPAIN Also Read: Top 70 Teradata Interview Questions The NVL2 function : Returns one of two specified values (arguments#2 and Arguments#3) based on weather the first arguments in NULL or not. It requires three arguments. The rule is as follows: If expression 1 is not null, expression 2 is returned If expression 1 is null, expression 3 is returned The best NVL2 example SELECT NVL2(NULL,'FRANCE','SPAIN'), NVL2('ENGLAND','FRANCE','SPAIN'
Read more