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

.
SELECT * FROM PAOLO.TORDER
WHERE ORDERNO BETWEEN 15000 AND 17000
STAGE 2
PREDICATES
1
INDEXED
predicate
root
page
DBAS
Buffer
Figure 3-24 Analysis of Stage 1 and Stage 2 predicates
Stage 1 predicates are simple predicates evaluated by DB2. They are evaluated first to
reduce processing cost and eliminate the number of rows to be evaluated by the complex
predicates. Stage 2 predicates are complex predicates evaluated by DB2’s more complex
analysis. They are also known as residual predicates.
An indexable predicate is a predicate that can use a (matching) index access as an access
path. Indexable predicates are always stage 1, but not all stage 1 predicates are indexable.
Stage 1 predicates are better than stage 2 predicates.
However, if your application has been coded in C on a work station, that language does not
have a DECIMAL data type, although some of the existing tables might have columns defined
as DECIMAL(p,s).
Java does not have a fixed length character string data type; every string is variable length.
66 DB2 UDB for z/OS Version 8 Performance Topics
STAGE 1
PREDICATES
2
OTHER index key
predicates applied
some
leaf
pages
TORDER
.
Applies ALL
Stage 2
Predicates
4
3
Applies ALL
other Stage 1
predicates
some
data
pages
I/O
ROWS
PAGES
GETPAGE results in
I/O operation
if the page is not in
the BUFFERPOOL.
GETPAGES
Scans DATA
PAGES from
TORDER doing
matching index
scan from
15000 to 17000