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MD5 hashes are commonly used with smaller strings when storing passwords, credit card numbers or other sensitive data in databases such as the popular MySQL. This tool provides a quick and easy way to encode an MD5 hash from a simple string of up to 256 characters in length. MD5 hashes are also used to ensure the data integrity of files. Mar 19, 2012 The hashing algorithm will generate the same surrogate keys on different Teradata systems but key collisions can happen. Therefore, the hashing algorithm needs to generate a uniqueness value as part of the surrogate key generation process to eliminate key collisions. CSUM and ROWNUMBER comparison in Teradata - Example script with statistics and syntax - Surrogate key generation in Teradata We keep getting questions about which one to use for sequence number generation in Teradata. Many people use CSUM(1,1) as its easier and other databases support that. But, in Teradata, it is not recommended. The checksum setting applies to primary data rows, fallback data rows, and all secondary index rows for the table. This feature detects and logs disk I/O errors: it does not fix them. The more words used to generate the checksum value, the better able that checksum is to detect disk I/O errors.
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A row is assigned to a particular AMP based on the primary index value. Teradata uses hashing algorithm to determine which AMP gets the row.
Following is a high level diagram on hashing algorithm.
Following are the steps to insert the data.
- The client submits a query.
- The parser receives the query and passes the PI value of the record to the hashing algorithm.
- The hashing algorithm hashes the primary index value and returns a 32 bit number, called Row Hash.
- The higher order bits of the row hash (first 16 bits) is used to identify the hash map entry. The hash map contains one AMP #. Hash map is an array of buckets which contains specific AMP #.
- BYNET sends the data to the identified AMP.
- AMP uses the 32 bit Row hash to locate the row within its disk.
- If there is any record with same row hash, then it increments the uniqueness ID which is a 32 bit number. For new row hash, uniqueness ID is assigned as 1 and incremented whenever a record with same row hash is inserted.
- The combination of Row hash and Uniqueness ID is called as Row ID. 512 bit encryption key generator.
- Row ID prefixes each record in the disk.
- Each table row in the AMP is logically sorted by their Row IDs.
How Tables are Stored
Tables are sorted by their Row ID (Row hash + uniqueness id) and then stored within the AMPs. Row ID is stored with each data row.
Row Hash | Uniqueness ID | EmployeeNo | FirstName | LastName |
---|---|---|---|---|
2A01 2611 | 0000 0001 | 101 | Mike | James |
2A01 2612 | 0000 0001 | 104 | Alex | Stuart |
2A01 2613 | 0000 0001 | 102 | Robert | Williams |
2A01 2614 | 0000 0001 | 105 | Robert | James |
2A01 2615 | 0000 0001 | 103 | Peter | Paul |
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CREATE TABLE command is used to create tables in Teradata.
Syntax
Following is the generic syntax of CREATE TABLE statement.
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![Chesum Key Generation In Teradata Chesum Key Generation In Teradata](https://techbros.com.tr/wp-content/uploads/2019/11/datastage.png)
- Table Options − Specifies the physical attributes of the table such as Journal and Fallback.
- Column Definition − Specifies the list of columns, data types and their attributes.
- Index Definition − Additional indexing options such as Primary Index, Secondary Index and Partitioned Primary Index.
Example
The following example creates a table called employee with FALLBACK option. The table contains 5 columns with EmployeeNo as the Unique Primary Index.
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Once the table is created, you can use SHOW TABLE command to view the Definition of the table.