Tomasz Ostrowski <tometzky+pg(at)ato(dot)waw(dot)pl> writes:
> I've noticed that hash indexes can't currently (in PG10) be multicolumn.
> Are they technically hard to implement or just nobody took such a feature?
It's not simple, particularly not if you wish that the index would support
queries specifying conditions for just a subset of the indexed columns
(an assumption that's buried pretty deeply in the planner, for one thing).
Then you couldn't compute the hash.
regards, tom lane