I have written before about links that tap into indexes and how they can speed things up. Especially when you can hit ALL of the fields in the index.
This week I was troubleshooting a report that took 2 hours to run and found a similar case. The report was from a Sage/MAS accounting application. I saw a link between invoiceHistoryHeader and InvoiceHistoryDetails where it took 2 fields to make a unique match. I checked the index tags for the primary key (red colored tabs) and and found that there were 3 fields in the details table primary index while the header table had only 2. Since we only had two of those fields to use for linking I wanted to make sure the link went from the details to the header, so that the link would completely hit the index. From the arrangement of the tables that appeared to be true, but when I hit “Auto Arrange” I could see that the join actually started with the header and went to the details.
Reversing the join allowed the report to complete in 7 minutes. Still slow, but a huge improvement over 2 hours.