Understanding the Calculator Cache Bitmap

For the calculator cache, Essbase separates sparse dimensions in the database into two groups:

Essbase starts with the first sparse dimension in the database outline and fits as many sparse dimensions as possible into the bitmap. The dimensions that fit are the bitmap dimensions. Essbase stops the process when it cannot fit another complete sparse dimension into the bitmap. Because the calculator cache controls the size of the bitmap, the number of sparse dimensions that can fit in the bitmap depends on the size of the calculator cache (and the number and size of the sparse dimensions).

The remaining sparse dimensions are the anchoring dimensions. For anchoring dimensions, Essbase cannot use the bitmap to determine whether blocks exist.

To see which dimensions are anchoring dimensions and which are bitmap dimensions, use the SET MSG DETAIL calculation command to display bitmap information in the application log.

Carefully order the sparse dimensions in your outline so that as many dimensions as possible can be placed into the bitmap. Start with the dimension that contains the fewest members and continue until the dimension with the most members is last. This order allows more dimensions to fit into the bitmap and results in improved calculation performance.

Note:

The order of sparse dimensions in the outline also affects query performance. See Optimizing Query Performance.

Essbase uses a single bitmap if there are multiple anchoring dimensions or if the calculator cache is not large enough to support multiple bitmaps, and uses multiple bitmaps if there is one anchoring dimension.

A single bitmap has these properties:

Multiple bitmaps have these properties:

Essbase chooses one of three options, as shown in Table 175, for the calculation:

Table 175. Calculator Cache Options

Option

Method

Performance Rating

1

Single anchoring dimension, multiple bitmaps

1

2

Single anchoring dimension, single bitmap

2

3

Multiple anchoring dimensions, single bitmap

3

Essbase chooses the optimal performance method for a database calculation, based on the size of the calculator cache. If the calculator cache size is too small for any of the above options, Essbase does not use a calculator cache. Calculation performance may be significantly impaired.

Enabling parallel calculation may change which calculator cache option is used. See Calculator Cache.

Caution!

If you are calculating the database for the first time, the size of the calculator cache is particularly significant for calculation performance. If possible, ensure that the calculator cache is large enough for Essbase to use the optimal calculator cache option.