Bit mapping is a technique used by the Optimizer to effectively link several weakly selective indexes in a way that creates a result that drastically reduces the number of base rows that must be accessed to retrieve the desired data. The process determines common rowIDs among multiple NUSI values by means of the logical intersection operation.
Bit mapping is significantly faster than the three-part process of copying, sorting, and comparing rowID lists. Additionally, the technique dramatically reduces the number of base table I/Os required to retrieve the requested rows.
For more information about:
- When Teradata Database performs NUSI bit mapping, see Database Design, B035-1094.
- How NUSI bit maps are computed, see Database Design, B035-1094.
- Using the EXPLAIN modifier to determine if bit mapping is being used for your indexes, see Database Design, B035-1094 or SQL Data Manipulation Language , B035-1146 .