Bitmap indexes use bit arrays (commonly called bitmaps) and answer queries by performing bitwise logical operations on these bitmaps.
Unlike other indexing methods, SAP Sybase IQ indexes are based on the cardinality of the data and the way the data will be used. SAP Sybase IQ has indexes for both low- and high-cardinality columns.
Bit-wise indexing benefits both low- and high-cardinality columns. It translates the distinct data elements of a column into a bit mask.
Cardinality is the number of unique values in a column:
- Low-cardinality data: fewer than 1500 unique values. Examples are days of the week, gender and states.
- High-cardinality data: 1500 unique values or more. Examples are primary keys, telephone numbers, social security numbers.
Example of low-cardinality bit-wise indexing
The table on the left represents unindexed data. The table on the right represents
the same data as a bit-wise index.
Note: These is not an actual index, but merely a
representation of how bit-wise indexing works for low-cardinality
data.
Example of high-cardinality bit-wise indexing
The data is stored vertically.
