I have a query activity in which the SQL validates, but it doesn't populate the target data extension. I created another query activity with a simpler query that does populate the extension. I also modified the first query activity using a TOP N clause. This works. Before I go through a binary search of the records to find the offending row (by iteratively changing N in the TOP N clause and re-running the QA), is there a log or other repository in which I can see the actual error causing the QA to fail?
The amount of data concerned is very small ~ 250 records at this point. I'd be very surprised if it was a timeout. After some fiddling, it appears that it works if the QA is set to Overwrite, but not if it's set to Update. The target Data Extension has a primary key and the field from which it's populated contains unique values.