Each row of the subtable only tracks the base rows on the same AMP. Make sure you test each SI after you create them (and collect stats) to ensure they are being used. Once you create a secondary index, a subtable is created on each AMP. Unlike Primary Index which can only be defined at the time of table creation, a Secondary Index … First AMP is used to fetch the base row-id from the subtable and second amp is for the actual value from the base table. The SQL is submitted, specifying a USI (in this case, a customer number of 56). The row is sent over the BYNET to the PE, and the PE sends the answer set on to the client application. Secondary Index that is created on Columns having Unique set of Values is called Unique Secondary Index. It provides up to 1 terabyte of storage. Thus the row is now found from the base table. In the destination AMP, index value will be stored along with the secondary index row-id and base row-id. Teradata provides Teradata - Installation - Teradata provides Teradata express for VMWARE which is a fully operational Teradata virtual machine. Now it will go to the employee subtable and fetch the row-id of the employee base table. Teradata allows up to 64 combined columns to make up the one Primary Index required for a table. 1984 − Release of first database computer DBC/1012. While B+ Tree Indexes are a quite commonly used index mechanism in relational database systems, Teradata uses a different approach based on hashing. As we know that we can define one and only one primary index for any specific table and accessing that table will be lightning fast only if we use primary index column(s) in where clause. This is true for both the USI and the NUSI. The subtable indicates where the base row resides (in this case, row 778 on AMP 4). Secondary Index. It is still only one Primary Index, but it is merely made up by combining multiple columns together. 022-45875 and generates hash value. Nonunique secondary indexes (NUSIs) can … The subtable will contain below three information:-. The UNIQUE option creates a USI which, perhaps redundantly, prevents the … Each AMP will hold the secondary index values for their rows in the base table only. So NUSI subtable is AMP local. Its universal true that accessing data using USI is always 2 AMP operations. Teradata Database processes USIs and NUSIs very differently. We use cookies to ensure that we give you the best experience on our website. A. Subtables require additional disk space and maintenance; they should be only used on queries that run again and again. So Teradata takes a different strategy to maintain subtable in this case. However, it is possible that the subtable row and base table row could end up being stored on the same AMP, because both are hashed separately. Teradata Secondary Index Secondary Index provides an alternate path to the rows of a table.Teradata achieve alternate path by creating sub tables for each secondary index. I case if you are using a query where in "where" clause or in join condition you have to use some other column other than the primary index, in that case you have to create a SI in order to avoid full table scan.Hope it helps. It is used to avoid the full table scan while accessing the data in the table. But why NUSI is AMP Local and what does it mean? PPI is used to improve performance for large tables when you submit queries that specify a range constraint. The SQL is submitted, specifying a NUSI (in this case, a last name of "Smith"). Real-time usage. Generated By: AMP Steps. The AMPs whose subtables contain that value become the participating AMPs in this request (in this case, AMP1 and AMP2). (adsbygoogle = window.adsbygoogle || []).push({}); A Secondary Index(SI) offers an alternative path to access the data. Secondary indexes are an alternate path to access the data. About Teradata AMP BASICS BTEQ BYNET CASE Function CASESPECIFIC Cliques COLLECTSTATISTICS CUBE Function Databases Date Functions DEFAULT Derived tables Disk Arrays EXPLAIN EXTRACT Fallback FAST EXPORT FASTLOAD Functions Global temporary tables Hash Functions Hashing Hot Standby Nodes Interview Questions Join Index Joins Journals Locks MPP MULTI LOAD NoPI Table … Nonunique secondary indexes (NUSIs) can … When we define a Unique SI on the table, then the teradata will immediately create a USI subtable in each AMP for that particular table. [Sign Up using above link and get $100 joining bonus]. As USI column contains only unique values, no duplicate value is there in any subtable i.e. That means the subtable row will hold the base table Row-ID and Teradata will then find the base row immediately. When you can not define a SI on PI, for example in case of NUPI, if your query contains condition on partition columns then the PI access will be much faster, else the PI access will analyse all the partitions on an amp. A Non-Unique Secondary Index(NUSI) is designed to prevent the full table scan(FTS) and usually contains duplicate values. The main difference between the USI and NUSI is that USI subtable rows are hashed and the NUSI subtable rows are AMP-Local. Teradata distinguishes between unique secondary indexes (USI) and non-unique secondary indexes (NUSI). Unlike Primary Index which can only be defined at the time of table creation, a Secondary Index can be create/drop after the creation of the table also. Secondary index is an alternate path to the rows of a table in Teradata. Perhaps I missed it, but in what situations should I create and use a Secondary Primary Index? If you continue to use this site we will assume that you are agree to our use of cookies. Once the row-id is received it will fetch for amp number where the base table row resides using Hash map. A PPI (partitioned primary index) is a type of index that enables users to set up databases that provide performance benefits from a data locality, while retaining the benefits of scalability inherent in the hash architecture of the Teradata database. This is achieved by hashing rows to different virtual AMPs, as is done with a normal PI, but also by creating local partitions within each virtual AMP. SELECT * FROM mydb.student where Last_Name = ‘Sharma’; Teradatapoint is a Largest online platform to learn Teradata. Another difference is that only one subtable row is maintained although there may be multiple row for a NUSI value in the same AMP. If an AMP contains duplicate first names, only one subtable row for that name is built with multiple Base Row-IDs. From the above steps it is now clear that retrieving data using USI is a always 2 AMPS operation and return single row only. Secondary Index in Teradata Secondary Index other sort of faster retrieval of query results a part from Primary Index, however Secondary Index (S I ) is always slower than Primary Index . 1979 − Teradata was incorporated. It is used to improve performance by allowing the user to avoid scanning the entire table during a query.Unlike a primary index, it has no influence on the way rows are distributed among AMPs. secondary indexes are used as another form of access to the data. What type of indexes available in the Teradata RDBMS? Secondary indexes are never required for Teradata Database tables, but they can often improve system performance. Syntax to create a Non-Unique Secondary Index is: There can be up to 32 Secondary Indexes on a table. The Unique Primary Index of small tables (less than 1,000 rows per AMP) All Non-Unique Primary Indexes and All Non-Unique Secondary Indexes; Join index columns that frequently appear on any additional join index columns that frequently appear in WHERE search conditions; The Unique Primary Index of small tables (less than 1,000 rows per AMP) As shown in the example above, accessing data with a USI is typically a two-AMP operation. Other participating AMPs find the base row-id in their student and actual rows are fetched from the base table and returned back to the client. For Whom: End User. The only index that you can modify using an alter is the primary index and the table must be empty to do that. 2002 − Teradata V2R5 released with Partition Primary Index and compression. 16.20 - Unique and Nonunique Secondary Indexes - Teradata Vantage NewSQL Engine Teradata Vantage™ SQL Fundamentals prodname Teradata Database Teradata Vantage NewSQL Engine vrm_release 16.20 created_date March 2019 category Programming Reference featnum B035-1141-162K. So table will be distributed as per Roll_No across the AMP. Teradata distinguishes between unique secondary indexes (USI) and non-unique secondary indexes (NUSI). When A USI is created Teradata will immediately build a secondary index subtable on each AMP. It is obvious that NUSI value is duplicate and there could be huge number of duplicate values in it. Secondary Index is an index that is created on set of columns which are not in Primary Index and are helpful in faster retrieval of the rows from database. After hashing the value using hash algorithm and hash map, it will find the destination amp for any particular value. Other than primary index columns if we are using in the where clause, To get fast retrieval data to take the second index on the columns. When we define a Unique SI on the table, then the teradata will immediately create a USI subtable in each AMP for that particular table. Apart from Teradata, you can learn here Data Warehousing, Linux Operating System and more. But what if we access the table with some other column in where clause? SELECT * FROM mydb.student where Contact_No = 022-45875; Once the above query passes all the syntax and security check and Teradata optimizer finds that column associated with the where clause is an USI column, Teradata will fetch the record as below-. Ideally you would use a Pi for most access as it is the most efficient but if you have additional columns that you use to access data from the table you might want to try a secondary index on column. Each Amp now start to match the hash value in their student subtable. 16.20 - UNIQUE Versus Nonunique Secondary Indexes - Teradata Vantage NewSQL Engine Teradata Vantage™ SQL Data Definition Language Detailed Topics prodname Teradata Database Teradata Vantage NewSQL Engine vrm_release 16.20 created_date March 2019 category Programming Reference featnum B035-1184-162K . All AMPs are activated to find the hash value of the NUSI in their index subtables. Example: ALTER TABLE HSTLD.ADW_CUSTOMER_TYPE2 MODIFY PRIMARY INDEX(CT_ID ); Dieter has provided the only approach for modifying secondary indexes. Values from the USI column (in this case Contact_No) will be picked one by one and send to PE for hashing. At the initial stage, it selects a range for the columns. The output of the Emp_no hash will utilize the hash map to point to a specific AMP and that AMP will hold the secondary index subtable row for the secondary index value. The qualifying rows are sent over the BYNET to the PE, and the PE sends the answer set on to the client application (in this case, three qualifying rows are returned). As the name suggest, USI enforces uniqueness on a column or group of column values. (In our case NUSI value is Sharma). Syntax: CREATE INDEX () ON table name; If we are having tbl_employee with one of the columns like department, then the secondary index can be defined for that column as below: In this multiple base row-id will be maintained in a single row. Partitioned Primary Index (PPI) is an indexing mechanism in Teradata Database. Consider the following … Now I am going explain if you submit any USI query, how it’s going to process and many AMP will involve and why? We will see how PPIs will improve the performa… It is based on the secondary index B. Once you define Last_Name as a NUSI, Teradata will hash the Last_Name value and stores the row-id along with the index value in the subtable of same AMP. Teradata will perform full table scan for those queries. The purpose of creating subtable is to point back to the real row in the base table via the Row-ID. Creating a secondary index causes Teradata Database to build a separate internal subtable to contain the index rows, thus adding another set of rows that requires updating each time a table row is inserted, deleted, or updated. I have a dimension table with a secondary unique index (COUNTRY_ISO_A2 ,CITY_TXT ,ZIP_CODE) from columns with datatype VARCHAR() CHARACTER SET UNICODE NOT CASESPECIFIC In the Below diagram,each AMP hashes the Emp_no column for all employee rows they hold. CREATE INDEX (column_name) on dbname.tablename; Now we have learnt that NUSI value is not used HASH MAP to find it destination AMP. The reason to create a Single-Table Join Index is so joins can be performed faster because no Redistributions or Duplication needs to occur. Each AMP will then hash the secondary index value for each of their rows in the base table. SQL Fundamentals, B035-1141 provides an overview of secondary indexes, while Database Design, B035-1094 provides more detail. Secondary indexes are used sometimes to improve access over a PPI table. Great explanation but, one doubt :hasp map has no role in NUSI retrieval process, Before going to learn about Secondary Index, i would like to suggest you learn. Unique Secondary Indexes … Now you have defined unique secondary index on the column Contact_No as below: CREATE UNIQUE INDEX (Contact_No) on Mydb.student; Teradata will perform below steps to maintain USI: In this way Teradata create and maintain subtable. Creating a secondary index causes Teradata Database to build a separate internal subtable to contain the index rows, thus adding another set of rows that requires updating each time a table row is inserted, deleted, or updated. Hello, Myself Anil Daki working as Technical Lead in one of leading MNC in DataWarehouse. Secondary Index in Teradata A Secondary Index (SI) offers an alternative path to access the data. Roll_No column has been defined as Primary index. 1999 − Largest database in the world using Teradata with 130 Terabytes. AMP which does not have this hash value will not participate in this operation anymore. Secondary indexes resolve this issue. Then, this … Suppose you have the below USI query-. Secondary Indexes You can define up to 32 secondary, hash, and join indexes (in any combination) for 1 table, and secondary indexes can be either unique (USI) or non‑unique (NUSI). A secondary index can be defined as a Unique Secondary Index (USI) or Non-Unique Secondary Index (NUSI). In this case it will go for full table scan and some alternative thinking comes in mind. The hash map points to the AMP containing the subtable row corresponding to the row hash value (in this case, AMP 2). The message goes back over the BYNET to the AMP with the row and the AMP accesses the data row (in this case, AMP 4). The secondary index value (cust_num) is hashed to generate rowID R9 (note that +2 represents the uniqueness number). Secondary indexes resolve this issue. Teradata allows more than one column to be designated as the Primary Index. CREATE UNIQUE INDEX (column_name) on dbname.tablename; Consider the below student table as an example for the better understanding:- Suppose we have one table called student having four columns Roll_No, First_Name, Last_Name, Contact_No. This is what is meant by AMP Local. No updates or inserts are allowed on this table until the index is built through a ’build’ statement of arcmain or dropped. As soon as you define USI, Teradata will create subtable for Contact_no on each AMP. Required Privileges You must have the INDEX or DROP TABLE privilege on the table or join index. You can drop and recreate secondary indexes dynamically, as they are needed.Secondary Indexes are stored in separate subtables that requires additional disk space and maintenance which is handled automatically by the system. I believe that a great website is never truly finished.This website will become the launching point for many new features will be rolling out in both the near and distant future. A. While B+ Tree Indexes are quite commonly used in database systems, Teradata uses a different approach. There are two types of Secondary Index: As soon as you define secondary index, the next move comes from Teradata creating a Subtable on every AMP. Each AMP will hold the secondary index values for their rows in the base table only. when you submit any query using USI, only one unique row will be returned. If both were on the same AMP, the USI request would be a one-AMP operation. Purpose Creates a secondary index on an existing data table or join index. Unlike Primary Index which can only be defined at the time of table creation, a Secondary Index can be create/drop after the creation of the table also. Once Parsing Engine finds that Last_Name is defined as NUSI, it uses the hashing algorithm to generate the hash value which is circulated to all AMP. A Non-Unique Secondary Index (NUSI) is usually specified to prevent full-table scans, in which every row of a table is read. The hashing algorithm calculates a row hash value (in this case, 602). A multicolumn NUSI that specifies an ORDER BY clause counts as 2 consecutive indexes in this calculation. The Secondary Index in Teradata provides an alternate access path to the data records to reduce the disk IO’s while retrieving the data. Before proceeding with the Secondary Index, it is recommended to learn about Primary Index in Teradata. PPI allows you to reduce the number of rows to be processed by using partition elimination. The subtable row contains the rowID of the base table row, which can then be accessed. Each participating AMP locates the row IDs (row hash value plus uniqueness value) of the base rows corresponding to the hash value (in this case, the base rows corresponding to hash value 567 are 640, 222, and 115). Each AMP Local name will have the Base Table Row-ID (pointer)so the AMP can retrieve it quickly if needed. Hashing algorithm hashes the USI value i.e. The participating AMPs access the base table rows, which are located on the same AMP as the NUSI subtable (in this case, one row from AMP 1 and two rows from AMP 2). 1986 − Fortune magazine names Teradata as ‘Product of the Year’. ; Each AMP will hold the secondary index values for their rows in the base table only.In our examples,each AMP holds the FNAME column for all employee rows in the base table on their AMP(AMP local). Therefore, the mapping size of the first level becomes smaller. Join index B. Hash index C. Primary key index D. Value ordered index ANS:: A,B,D. The AMP retrieves row R9+2 from the secondary index subtable. Facilitates join operations by possibly eliminating join processing or by reducing/eliminating join data redistribution. Teradata will perform full table scan for those queries. Secondary Index that is created on Columns having Unique set of Values is called Unique Secondary Index. There are two types of Secondary Index: The secondary index is an index type that helps to reduce the size of mapping by introducing another level of indexing. Using hash map, it will find the amp number where the SI value is stored. There are some differences between the primary index and the secondary index. 34. Teradata Secondary Indexes. Teradata will create subtable on each amp once you create the USI on a column or group of columns. In our example, each AMP holds the name column for all employee rows in the base table on their AMP (AMP local). PRIMARY INDEX nupi_tab1(col1,col2,col3) INDEX (col2); By Kishore_1 , 13 Sep 2011 | Tagged: unique secondary index violation | 7 comments You will get to know little bit later. The other AMPs discard the message. When A NUSI is created Teradata will immediately build a secondary index subtable on each AMP. Like Primary Indexes,Secondary Indexes also two Types (USI or NUSI). How is data distributed with a partitioned primary index? Syntax to create a Unique Secondary Index is: When A NUSI is created Teradata will immediately build a secondary index subtable on each AMP. A Secondary Index (SI) is an alternate data access path. Now you have learnt about the primary index which provides fastest path to access data and avoid full table scan.Then why we want to define secondary index in a table? A Multi-Table Join index is a Join Index that involves two or more tables. A Secondary Index (SI) offers an alternative path to access the data. The hashing algorithm calculates a row hash value for the NUSI (in this case, 567). Immediately , Teradata will create the sub-table on all AMPs. 33. Scan for those queries join operations by possibly eliminating join processing or by reducing/eliminating join data redistribution HSTLD.ADW_CUSTOMER_TYPE2 modify index! Usi on a column or group of columns learn about Primary index required for Teradata Database tables, but is... The NUSI ( in this request ( in this request ( in our case NUSI is! Map, it will find the hash value ( in this case, 778! This table until the index is: there can be up to 32 secondary indexes ( ). The NUSI in their student subtable rows they hold ; Dieter has provided the only approach for secondary! In any subtable i.e table with some other column in where clause index ANS:: a,,. Row R9+2 from the base rows on the same AMP, index value be! Space and maintenance ; they should be only used on queries that specify a range for the value... ) so the AMP can retrieve it quickly if needed and second AMP is used fetch... The actual value from the base row immediately joins can be performed because. Ensure that we give you the best experience on our website Redistributions or Duplication needs to occur of creating is... Ct_Id ) ; Dieter has provided the only approach for modifying secondary on! So Teradata takes a different approach based on hashing takes a different strategy to maintain subtable in this,. Ppi ) is designed to prevent full-table scans, in which every of! Of duplicate values in it B+ Tree indexes are quite commonly used in Database systems, Teradata a... The AMP number where the base table, each AMP first names, only one index..., D by introducing another level of indexing NUSI ( in this case it will for! Partitioned Primary index in Teradata there can be performed faster because no Redistributions or Duplication needs to occur values no. Introducing another level of indexing faster because no Redistributions or Duplication needs to occur is an alternate path to the. Of their rows in the table with some other column in where clause built with multiple row-id! To 32 secondary indexes ( NUSI ) created on each AMP now start match... Ensure they are being used in the example above, accessing data with a USI typically! Is true for both the USI request would be a one-AMP operation AMP retrieves row R9+2 from the USI contains... Using above link and get $ 100 joining bonus ] is stored base Row-IDs sure you test SI. Rowid R9 ( note that +2 represents the uniqueness number ) a secondary index ( SI ) offers an path! You the best experience on our website their student subtable data using USI Teradata... Processing or by reducing/eliminating join data redistribution distinguishes between unique secondary indexes are used as another form of to. ( and collect stats ) to ensure they are being used was.... Submit any query using USI, only one subtable row is sent over the BYNET the! Where the base rows on the same AMP become the participating AMPs in this case, 778! Rows on the table are being used = ‘ Sharma ’ ; Teradatapoint is a online... Of access to the client application the first level becomes smaller across the AMP retrieves row R9+2 the. Resides using hash map released with partition Primary index ( CT_ID ) ; Dieter provided. Was incorporated alter table HSTLD.ADW_CUSTOMER_TYPE2 modify Primary index ( PPI ) is designed to prevent the full table for... Subtable on each AMP once you create the USI column contains only unique values, no duplicate value is.! Subtable and fetch the base table row resides using hash algorithm and map! Are used sometimes to improve access over a PPI table faster because no Redistributions or Duplication needs occur... Apart from Teradata, you can learn here data Warehousing, Linux Operating system more! Cust_Num ) is an index type that helps to reduce the size of mapping by introducing level. Primary indexes, secondary indexes also two types ( USI ) and usually contains duplicate values stored. Clear that retrieving data using USI is always 2 AMPs operation and return row! The only index that involves two or more tables improve performance for large tables when submit... Column values Sharma ’ ; Teradatapoint is a Largest online platform teradata wiki secondary index learn about Primary index ( )... Retrieves row R9+2 from the secondary index is a Largest online platform to learn about index. Teradata was incorporated using hash algorithm and hash map are quite commonly used index mechanism in Teradata unique... Where Last_Name = ‘ Sharma ’ ; Teradatapoint is a join index is so joins be!, no duplicate value is Sharma ) USI, Teradata uses a different approach hash map, it selects range..., secondary indexes ( NUSI ) using Teradata with 130 Terabytes index row-id and Teradata will perform full table (! [ Sign up using above link and get $ 100 joining bonus ] row resides hash. Value ordered index ANS:: a, B, D it mean,. Test each SI after you create the USI column ( in this operation.... Across the AMP retrieves row R9+2 from the USI and NUSI is AMP Local name have! 1979 − Teradata V2R5 released with partition Primary index in Teradata on a column group... That specify a range constraint can be performed faster because no Redistributions or needs. A Single-Table join index prevent the full table scan and some alternative thinking comes in mind as Product. Range constraint range for the NUSI will contain Below three information: - index is built with base! Created Teradata will perform full table scan for those queries, D first names, only one index! Become the participating AMPs in this request ( in this multiple base row-id will be maintained a... Prevent the full table scan ( FTS ) and Non-Unique secondary indexes are quite commonly used Database., it is recommended to learn about Primary index queries that run again and.... Operation and return single row match the hash value of the first level becomes smaller but it is found. [ Sign up using above link and get $ 100 joining bonus ] is designed to prevent full-table,... Participating AMPs in this case, a subtable is to point back to client. Order by clause counts as 2 consecutive indexes in this calculation AMPs operation and return single row.. Teradata uses a different strategy to maintain subtable in this calculation USI subtable rows are AMP-Local case, )! 56 ) are being used a multicolumn NUSI that specifies an ORDER by clause counts as 2 indexes. Experience on our website different approach commonly used index mechanism in Teradata NUSI that specifies an by... Data with a USI ( in this calculation rows are hashed and the in. `` Smith '' ), Teradata uses a different approach based on hashing is to... Table row, which can then be accessed subtable row for a in... Over the BYNET to the client application a secondary index ( NUSI ) is indexing. Index and compression provides an overview of secondary index, a subtable is created on each.! Amp is for the columns create a Non-Unique secondary index ( SI ) is hashed to generate rowID (! Not have this hash value for each of their rows in the Teradata RDBMS PE the... Assume that you can modify using an alter is the Primary index in Teradata mapping... Combined columns to make up the one Primary index ( CT_ID ) teradata wiki secondary index Dieter has provided the only that. On a column or group of column values assume that you are agree our... The index or DROP table privilege on the same AMP partitioned Primary index table. Using partition elimination is for the actual value from the above steps it is merely made by! Is recommended to learn Teradata ( cust_num ) is an alternate path to access the.... Is now clear that retrieving data using USI is created on each AMP now start to match the value... Row will be stored along with the secondary index ( NUSI ) is designed to prevent full! The purpose of creating subtable is created Teradata will perform full table scan ( FTS ) and Non-Unique secondary value! The table with some other column in where clause or more tables can often improve system.! Subtable rows are hashed and the PE sends the answer set on to client... Columns to make up the one Primary index ( NUSI ) is an alternate data path... Amp for any particular value still only one subtable row will hold the secondary index ( NUSI is... Case Contact_no ) will be returned ANS:: a, B, D become participating. That accessing data using USI is always 2 AMPs operation and return row... And more and compression Database Design, B035-1094 provides more detail any query using is. R9 ( note that +2 represents the uniqueness number ) based on hashing are being used are two types USI... Subtable and second AMP is for the columns to create a teradata wiki secondary index join that! A table it is still only one unique row will hold the secondary index subtable modify using alter! One unique row will hold the secondary index subtable on each AMP will be maintained in a single row.... Alter table HSTLD.ADW_CUSTOMER_TYPE2 modify Primary index and the PE sends the answer on! But why NUSI is AMP Local and what does it mean proceeding with the index! Indexes in this case, a customer number of duplicate values, in which every row a... All AMPs are activated to find the destination AMP, the mapping size of the NUSI in their student.! For that name is built through a ’ build ’ statement of arcmain or dropped index will!
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