Today I learned: #PostgreSQL does not support ordering in updates.
That might not seem necessary at first, but in the very specific case that you have an index on a table and want to update the index field on many rows (e.g. to move items up in a queue) then you can easily run into contraint violations with large amounts of rows.
The fix - which I don't really like - is deferrable constraints. 
#MySQL and some others support order by in updates 🤔
Personal server of Hannes Leutloff aka yeldiR