Performance Opportunities Teradata

The Teradata optimizer has always had options available to it when performing SQL. It always attempts to use the most efficient path to provide the answer set. This is true for aggregation as well.

When performing aggregation, the main shortcut available might include the use of a secondary index. The index row is maintained in a subtable. This row contains the row ID (row hash + uniqueness value) and the actual data value stored in the data row.

Therefore, an index row is normally much shorter than a data row. Hence, more index rows exist in an index block than in a data block.

As a result, the read of an index block makes more values available than the actual data block. Since I/O is the slowest operation on all computer systems, less I/O equates to faster processing. If the optimizer can obtain all the values it needs for processing by using the secondary index, it will. This is referred to as a "covered query."

The creation of a secondary index is covered in this book as part of the Data Definition Language (DDL).



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