Optimizing grouping by eliminating grouping columns

 

All of the grouping columns are evaluated to determine if they can be removed from the list of grouping columns. Only those grouping columns that have isolatable selection predicates with an equal operator specified can be considered. This guarantees that the column can only match a single value and will not help determine a unique group.

This processing is done to allow the optimizer to consider more indexes to implement the query and to reduce the number of columns that will be added as key columns to a temporary index or hash table.

The following example illustrates a query where the optimizer might eliminate a grouping column.

    DECLARE DEPTEMP CURSOR FOR       SELECT EMPNO, LASTNAME, WORKDEPT         FROM CORPDATA.EMPLOYEE         WHERE EMPNO = '000190'
        GROUP BY EMPNO, LASTNAME, WORKDEPT

OPNQRYF example:

OPNQRYF FILE(EMPLOYEE) FORMAT(FORMAT1)
   QRYSLT('EMPNO *EQ ''000190''')
   GRPFLD(EMPNO LASTNAME WORKDEPT)

In this example, the optimizer can remove EMPNO from the list of grouping columns because of the EMPNO = '000190' selection predicate. An index that only has LASTNAME and WORKDEPT specified as key columns can be considered to implement the query and if a temporary index or hash is required then EMPNO will not be used.

Even though EMPNO can be removed from the list of grouping columns, the optimizer might still choose to use that index if a permanent index exists with all three grouping columns.

 

Parent topic:

Grouping optimization