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

► Reduced improvement when merge pass is still required
► Reduced improvement when query has two or more parallel sort composites to be merged
Multiple table GROUP BY sort
Let us look at the query in Example 3-8 where DB2 executes a parallel sort when grouping 4
million rows in 600,000 groups in a join of two tables.
Example 3-8 Multiple table GROUP BY sort query
SELECT C_CUSTKEY, C_NAME,
SUM(O_ORDERKEY * 0.0001) AS REVENUE,
C_ACCTBAL, C_NAME, C_ADDRESS, C_PHONE, C_COMMENT
FROM CUSTOMER, ORDER
WHERE C_MKTSEGMENT = 'BUILDING'
AND C_CUSTKEY = O_CUSTKEY
AND O_ORDERDATE < DATE('1994-12-05')
GROUP BY C_CUSTKEY, C_NAME,
C_ACCTBAL, C_NAME, C_ADDRESS, C_PHONE, C_COMMENT
Figure 3-35 shows the performance improvement.
Multiple table GROUP BY sort
Group By sort of 4M rows to 0.6M groups
nested loop join of 2 tables, 1 parallel group, DEGREE(30)
merge pass used for both V7 and V8
up to 46% reduction in elapsed time
no reduction in workfile activities
Fetch first 10
rows
Figure 3-35 Multiple table group by sort
Elapsed
secs
CPU
secs
Workfile
getpages
Fetch all rows Elapsed
secs
CPU
secs
Workfile
getpages
V7
Seq Sort
V8
Parallel
Sort
We observe, in the example of GROUP BY, the best situation is when we fetch the first 10
rows (reduction of 46% in elapsed time) when V8 is compared to V7.
Multiple table ORDER BY sort
Let us look at the query in Example 3-9 where the ORDER BY is requested.
Delta
54 29 -46%
73 72.6 -0.5%
195400 193610 -1%
102 85 -17%
120 124 +4%
255108 253304 -0.7%
Chapter 3. SQL performance 81