Question

Hi I had a DB2 Query as below

select count(*) as count from 
table_a,
table_b,
table c

where
b.xxx=234 AND
b.yyy=c.wedf 

Result SEt:

Count

618543562

For the above query i even tried with Count(1) but when i took the access plan, cost is same.

select count(1) as count from 
    table_a,
    table_b,
    table c

    where
    b.xxx=234 AND
    b.yyy=c.wedf 

Result SEt:

Count

618543562

Is there any other way to reduce the cost.

PS: b.xxx,b.yyy, c.wedf is indexed..

Thanks in advance.

Answer 1

I think one of the problem are statistics on the table. Did you execute Runstats? Probably, the data distribution or the quantity of rows that has to be read is a lot, and DB2 concludes that is better to read the whole table, instead of process an index, and then fetch the rows from the table.

It seems that both queries are taking the same access plan, and I think they are doing table scans.

Are the three columns part of the same index? or they are indexed separately? If they are part of different indexes, is there any ANDing between indexes in the access plan? If there is not ANDing with different indexes, the columns has to be read from the table in order to process the predicates.

The reason count(1) and count(*) are giving the same cost, is because both has to do a TableScan.

Please, take a look at the access plan, not only the results in timerons, but also the steps. Is the access plan taking the indexes? how many sorts is executing?

Try to change the optimization level, and you will see that the access plans change. I think you are executing with the default one (5)

If you want to force the query to take in account an index, you can create an optimization profile