Okay, let's revisit the performance on this:
--===== If the test table already exists, drop it
IF OBJECT_ID('tempdb..#mytable','U') IS NOT NULL
    BEGIN
    DROP TABLE #mytable;
    END;

--===== Create the...

I'm guessing that the error comes from somewhere else, unless there's a security context difference between the way it runs in production and the way you are running it.

We're missing a few critical details here.   When you say you ran an import of a flat file, exactly what process did you use?   You then say you tried to...

Connection settings.   Also look at  the actual execution plan in both situations and compare them.

One easy way is to use SSRS and a matrix where Customer is the row group and the Method of Delivery is the column group.   That would create a pivot...

This is not an Oracle forum.   It's a SQL Server forum.   There are a few folks here that ARE familiar with Oracle, but not necessarily at a level high enough...