Bitmap index (Universal percentage in OWC - take 2)
Creating a Bit array (also known as Bitmap) is an useful technique when analyzing data. Especially (but not necessarily) in case of low cardinality (small number of distinct values in column compared to the number of records in a table). For example, a column having two distinct values (male/female or yes/no status) could be optimized in storage and represented using values 0 and 1 only (bits).
A collection of such bits forms a bit array or a bitmap. Bitmap indexes use those arrays performing bitwise operations on them.
Almost all DSS and analytical systems use bitmap indexes, and so does Analysis Services. An example where it can be seen is SQL Server Profiler. Query Subcube event represents the bitmap of hierarchies and attributes being used in a query. Yet, what’s fine for computers might not be for humans, hence there’s also Query Subcube Verbose event which explains that in plain English. Kind of.
Tags: mdx