plsql - SQL Distinct keyword bogs down performance?

Question

I have received a SQL query that makes use of the distinct keyword. When I tried running the query it took at least a minute to join two tables with hundreds of thousands of records and actually return something.

I then took out the distinction and it came back in 0.2 seconds. Does the distinct keyword really make things that bad?

Here's the query:

SELECT DISTINCT
    c.username, o.orderno, o.totalcredits, o.totalrefunds,
    o.recstatus, o.reason 
FROM management.contacts c 
    JOIN management.orders o ON (c.custID = o.custID)
WHERE o.recDate > to_date('2010-01-01', 'YYYY/MM/DD')

Answer 1

Yes, as using DISTINCT will (sometimes according to a comment) cause results to be ordered. Sorting hundreds of records takes time.

Try GROUP BY all your columns, it can sometimes lead the query optimiser to choose a more efficient algorithm (at least with Oracle I noticed significant performance gain).