DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

Star join access path without a sparse index
Figure 3-17 shows the access path to execute a star join (before PQ61458, which introduced
the sparse index function also in V7).
100
Time NLJ NLJ Store Fact
Figure 3-17 Star join access path (before PQ61458)
Characteristics of the star join access path:
► Use of a multi-column fact table index during outside-in phase. Nested-loop join (NLJ) is
used.
► No index on the snowflake work file during the inside-out phase. Sort-merge join (SMJ) is
used.
This process can be efficient if the number of rows in the fact table composite involves a low
number of rows, but can be very expensive for a large fact table composite.
Sparse indexes on work files
Figure 3-18 shows the structure of a sparse index on the work file.
The development of sparse indexes on work files in DB2 V8 improves the performance of the
star join execution.
58 DB2 UDB for z/OS Version 8 Performance Topics
500 15,000,000 600 30,000
SMJ
Prod
SMJ
Custmr ...
Outside-in
phase
Fact table index
(Time,Store,...)
Expensive to sort large
Fact table composite
Inside-out phase
Sort merge join is
used to join
snowflake to Fact
table composites