Slightly over-aggressive join promotion in SQL for ordering

[Malcolm Tredinnick]

Right now, we promote any table join used in the ordering portion to an outer join if it potentially contains NULL values (the corresponding Django field has null=True). There are some cases where this is unnecessary. If we already used that join in the filter statements and didn't make it an outer join at that point, it's because we are already filtering out the null values, so an inner join still won't lose any results in the ordering.

Thus, we should only promote joins for ordering if we needed to add that join for the ordering (this means self.alias_refcount[...] == 1, or something else that's easily detectable).

Noting this so I don't forget. Don't want to introduce new bugs at the moment and it's a bit of an edge case, so it can wait until post-1.0.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Anssi Kääriäinen]

This one seems to be fixed in: 8c72aa237918e31a525022f72b22cac75451af86