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

While, for
or
SELECT ... FROM ... ORDER BY A DESC, B DESC
SELECT ... FROM ... ORDER BY A ASC, B ASC
DB2 still has to perform a sort.
Table 3-10 shows a set of test cases, in which we compare queries under V7 to V8. We
assume that an ascending index on EMPO has been created for table EMP. Notice that the
access type in the PLAN_TABLE is I in both directions.
Table 3-10 Access type and sort with backward index scan
Query DB2 Version Access type in
PLAN_TABLE
SELECT EMPNO FROM EMP
ORDER BY EMPNO ASC
SELECT EMPNO FROM EMP
ORDER BY EMPNO DESC
SELECT EMPNO FROM EMP
ORDER BY EMPNO ASC
SELECT EMPNO FROM EMP
ORDER BY EMPNO DESC
3.14 Multiple distinct
Prior to DB2 V8, DISTINCT cannot be used more than once in a subselect, with the exception
of its use with a column function whose expression is a column. That is, it is allowed to specify
multiple DISTINCT keywords as an exception on the same column like:
► SELECT COUNT(DISTINCT(C1)), AVG(DISTINCT(C1))
However, if you use DISTINCT keywords more than once specifying different columns, you
get SQLCODE -127 which means “DISTINCT IS SPECIFIED MORE THAN ONCE IN A
SUBSELECT” occurs. For instance, in V7 you cannot have the following statement:
► SELECT COUNT(DISTINCT(C1)), AVG(DISTINCT(C2))
With DB2 V8, you can use multiple DISTINCT keywords in the SELECT statement and
HAVING clause. With DB2 V8, the following queries are possible:
► SELECT DISTINCT COUNT(DISTINCT C1), SUM(DISTINCT C2) FROM T1
► SELECT DISTINCT COUNT(DISTINCT C1), COUNT(DISTINCT C2) FROM T1 GROUP
BY A3
► SELECT COUNT(DISTINCT C1), AVG(DISTINCT C2) FROM T1 GROUP BY C1
► SELECT COUNT(DISTINCT(A1)) FROM T1 WHERE A3 > 0 GROUP BY A2 HAVING
AVG(DISTINCT (A4)) > 1
In these cases in V7 you have to divide the statements into multiple SELECT statements for
each distinct operation of columns. From the performance point of view, you have to execute
multiple SELECT statements to retrieve the value in multiple columns.
108 DB2 UDB for z/OS Version 8 Performance Topics
Notes
V7 I Base case
V7 I Sort needed
V8 I Same as V7
V8 I Sort no longer
required