Saturday, January 15, 2011

How does Hashing happens in Teradata?

How does Hashing happens in Teradata?


* Hashing is the mechanism through which data is distributed and retrieved to/from AMPs.
* Primary Index (PI) value of a row is the input to the Hashing Algorithm.
* Row Hash (32-bit number) value is the output from this Algorithm.
* Table Id + Row Hash is used to locate Cylinder and Data block.
* Same Primary Index value and data type will always produce same hash value.
* Rows with the same hash value will go to the same AMP.

So data distribution depends directly on the Row Hash uniqueness; be careful while Choosing Indexes in Teradata.

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