SELECT DISTINCT slow, how can I speed up

Hello. I have a question about how I can structure a SQL query to make the results come back faster. I have experimented a bit and include some results here.

We have read-only SELECT access to tables for a payroll and absence system running Oracle 10.0.1 on (I think) a dedicated Windows 2003 Server in the IT department on the same site. We do not have privileges to UPDATE, DELETE, or create anymore indexes and if I did, it would probably invalidate our software support contract.

We have staff using Access 2003 as a front end to the Oracle tables for the purposes of ad hoc queries. These staff regularly enter queries across 5-6 tables with various bits added to the SQL to limit the number of rows returned and match criteria. All the rows starts to come back within a second of the SQL starting to run.

When the SQL is like this (I have anonymised the table names).

SELECT a1,a2,a3,a5,a10
 FROM table1, table2, table3, table4
  WHERE table1.empID=table2.empID

     AND table2.empID=table3.empID
     AND table3.empID=table4.empID
     AND table4.absenceT IN (15,19)
     AND table4.year=2008;

The rows start to come back immediately. Around 1700-1800 rows typically come back and there is a lot of network IO. If I type

SELECT COUNT(*) FROM
(
SELECT a1,a2,a3,a5,a10
 FROM table1, table2, table3, table4
  WHERE table1.empID=table2.empID

     AND table2.empID=table3.empID
     AND table3.empID=table4.empID
     AND table4.absenceT IN (15,19)
     AND table4.year=2008

it returns in 2.14seconds saying it found 1792 rows.

If I type SELECT DISTINCT a1,a2,a3,a5,a10

only 20-50 rows come back and the query takes 28 seconds and there is a lot less network IO.

I only want the 20-50 rows but I don't want to wait 28 seconds. As Oracle seems to find all 1792 rows in 2.14seconds, why does it take 28 seconds only to return the unique ones.

These are ad hoc queries so I am not trying to optimize a single query. My question is Is there a more quick way of generally returning a DISTINCT set of rows other than using DISTINCT in the SELECT statement.

Thank you
Barry Received on Thu Aug 14 2008 - 13:18:41 CDT