Archives of the TeradataForum

Message Posted: Wed, 01 Feb 2006 @ 16:09:58 GMT

	<Prev	Next>		<<First	<Prev	Next>	Last>>

Teradata would probably never use the secondary index to access the data unless you have at least 4 blocks of data on an AMP. If it has to read two index blocks to read two data blocks, it would be silly to use the index when reading 4 data blocks gives access to all the data rows. Reads are slow on ALL computer systems. Teradata uses V-disks to spread the I/Os to a great degree - but an I/O is an I/O an measured in milliseconds and not nanoseconds like memory. Reduce I/Os and you increase performance.

Teradata will also use an index block instead of a data block based on index rows being shorter than data rows. Therefore, fewer blocks = fewer reads = faster performance.

Just a consideration for having a NUSI that is never used for reading data, but instead of data for things like aggregation.

Michael Larkins
Certified Teradata Master
Certified Teradata SQL Instructor

	<Prev	Next>		<<First	<Prev	Next>	Last>>