Data in MDC tables is organized in blocks along extent boundaries according to 

the dimensions (clustering values). A block (extent) is a set of contiguous pages 

on disk, so access to these records is sequential and with minimal I/O 

operations. The page size is specified at table space creation.

In Figure 10 4 the extents' boundaries are defined by the two dimensions: 

Region

and 

Year

.

Benefits of MDC

These are the benefits of MDC:

Range scans on any dimension use clustered data access, because each 

block corresponds to a set of sequential parts in the table guaranteed to 

contain data having that dimension value.

Dimensions can be accessed independently from each other through their 

block indexes without compromising the clustering of any other.

Block index scans can be combined by 

AND 

and 

OR

 logical operations. The 

resulting scan also uses clustered data access.

Access to clustered data is much faster than access to data through 

clustering indexes (single dimensional data clustering), because with MDC 

there is one pointer per qualifying block of pages versus one pointer per 

qualifying row in single dimensional data clustering.

With a specified block ID from a block index, scans on that block are very 

efficient and much faster than accessing each row through row ID.

 Chapter 10. Advanced DB2 UDB features 

333