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

Sparse Index
T1
Binary Search
Figure 3-18 Sparse index on work file
NLJ
t1.c = t2.c
Key RID
With a sparse index there is a binary search to find the entry for the key searched in memory
and scans a few rows in the work file.
Sparse index on work file:
► Made available in DB2 V7 via APAR PQ61458
► Enhanced in DB2 V8, sort key and a 5 byte RID
► Sparse indexes are built in cache when they are sorted in join column order
► Cache up to 240 KB in memory in thread local storage pool
► Probed through an equal-join predicate
► New ACCESSTYPE='T' in PLAN_TABLE for sparse index access as well as for IMWF
► Nested-loop join can be used for inside-out phase of star join
Sparse index for star join
Figure 3-19 shows the use of a sparse index in the inside-out phase in a star join.
With the sparse index the access path can be changed to avoid sort-merge-join in the
inside-out phase:
► Use of a multi-column fact table index during outside-in phase
► Use of a sparse index during inside-out phase
RID
... ...
T2 (WF)
T2
(WF)
Sorted in t2.c order
As a result there is no longer the need to sort a large composite file (no sort work file space
requirement) and reduction of the overhead (merge and repartition) caused by sort.
Chapter 3. SQL performance 59