Why does SQL bring back all values in a single condition quicker?

Question

Hi All,
I've stumbled across an interesting yet frustrating problem with one of our Discoverer Reports. The problem unfortunately is related to performance, which can be a never-ending struggle, as we all know.
Scenario:
We have a report that contains multiple outer joins that retrieves information from other sub ledgers. The type of report is tabular and it has a single page item called period name. A parameter has been setup on the period name column, which allows users to specify which period they want to report on. This is a real time report transaction report.
Problem:
The report takes various times to run depending on the amount of periods selected via the parameters. Timings are shown below –
All Periods                         –  2 min 22 sec     (36 Periods selected – 7 have data)
Apr                              -  0 min 43 sec           (01 Periods)
Apr + May                         -  1 min 06 sec      (02 Periods)
Apr + May + Jun                         -  1 min 36 sec      (03 Periods)
Apr + May + Jun + Jul                     -  2 min 29 sec      (04 Periods)
Apr + May + Jun + Jul + Aug               -  3 min 15 sec      (05 Periods)
Question:
I’m finding it very hard to understand why the time taken to run the report for 5 periods IS NOT quicker than running the report for all periods because the condition/where clause would have been refined?
I can confirm that the Period column is indexed. I’m not going to post the execution plans, as each are 117 rows long. I’m rather hoping some of you might have experienced something similar, and therefore able to point me in the right direction.
Thanks,
Lance

Answer 1

Hi Rod,
Looking at the SQL generated in Discoverer this is what I get for All Periods:
Where ‘…….’
AND (o227124.i227127 IN ('MAR-07','FEB-07','JAN-07','DEC-06','NOV-06','OCT-06','SEP-06','AUG-06','JUL-06','JUN-06','MAY-06','APR-06','MAR-06','FEB-06','JAN-06','DEC-05','NOV-05','OCT-05','SEP-05','AUG-05','JUL-05','JUN-05','MAY-05','APR-05','MAR-05','FEB-05','JAN-05','DEC-04','NOV-04','OCT-04','SEP-04','AUG-04','JUL-04','JUN-04','MAY-04','APR-04','MAR-04','FEB-04','JAN-04','DEC-03','NOV-03','OCT-03','SEP-03','AUG-03','JUL-03','JUN-03','MAY-03','APR-03','MAR-03','FEB-03','JAN-03','DEC-02','NOV-02','OCT-02','SEP-02','AUG-02','JUL-02','JUN-02','MAY-02','APR-02','MAR-02'))
Statistics For All Periods
351 recursive calls
0 db block gets
     32301 consistent gets
61 physical reads
0 redo size
500 bytes sent via SQL*Net to client
13147 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
6 sorts (memory)
0 sorts (disk)
0 rows processed
And this is what I get for 6 Periods
AND (o227124.i227127 IN ('AUG-06','SEP-06','JUL-06','JUN-06','MAY-06','APR-06'))
Statistics For 6 Periods
351 recursive calls
0 db block gets
32301 consistent gets
7 physical reads
0 redo size
509 bytes sent via SQL*Net to client
11736 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
6 sorts (memory)
0 sorts (disk)
0 rows processed
Looking at the statistics the ‘All Periods’ query should take longer than the 6 periods, however this is not the case.
Thanks,
Lance
Message was edited by:
Lance Botha