At the same time this advice from
http://stackoverflow.com/questions/42179012/how-to-shuffle-array-in-postgresql-9-6-and-also-lower-versions
works, don't know why though:
words=> select array_agg(u order by random())
words-> from unnest(array['a','b','c','d','e','f']) u;
array_agg
---------------
{d,a,f,c,b,e}