On 8 Nov 2013, at 19:33, David Cantrell <da...@cantrell.org.uk> wrote: [...] > Because you might need to know which of two events at 2013-11-08Z19:31:04 > happened first. Sure you could use microseconds or whatever to get better > resolution, but all that does is make the problem less likely, it doesn't > make it go away. You also normally want sort order to be consistent. If you > have two records where the sort field is the same, the order they come out is > going to be unpredictable.
That's just a specific example of the general problem where one desires a stable sort by a non-unique column. The simple solution is to just add more columns to the ORDER BY until the tuples *are* unique. The primary key is an obvious choice of tie-breaker if you don't care about the order so long as it's consistent.
