[HACKERS] Multicolumn hash indexes

Hi.

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?

I think multicolumn hash indexes should help pretty significantly with 
queries like:
- where username=? and user_post_id=?
- where client_id=? and period=? and invoice_number=?
etc.

I imagine that calculating a multicolumn hash should be pretty 
straightforward to implement - after hashing bytes of first column just 
keep going and update the hash state with bytes of a second and 
subsequent columns. And it should allow for faster (O(1), less IO) and 
much smaller (better cached, less IO again) multicolumn indexes. Also in 
PG10 hash indexes are WAL-logged and therefore much easier to work with. 
What do you think?

-- 
Tomasz "Tometzky" Ostrowski


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