oracle diff: how to compare two tables?

Try this:

(select * from T1 minus select * from T2) -- all rows that are in T1 but not in T2
union all
(select * from T2 minus select * from T1)  -- all rows that are in T2 but not in T1
;

No external tool. No performance issues with union all.

You can use a tool like AQT to create diffs between tables.

Another approach would be to dump the tables to a text file and use a diff tool like WinMerge. With this approach, you can use complex SQL to turn the tables into the same layout, first.

You can try using set operations: MINUS and INTERSECT

See here for more details:
O'Reilly - Mastering Oracle SQL - Chapter 7 - Set Operations