-- One answered guess per user per asset per session-half slot pair.

ALTER TABLE market_history_prospective_answers
  DROP CONSTRAINT IF EXISTS market_history_prospective_answers_user_symbol_slot_key;

ALTER TABLE market_history_prospective_answers
  ADD CONSTRAINT market_history_prospective_answers_user_symbol_slot_key
  UNIQUE (assigned_user_id, symbol, older_slot_start, newer_slot_start);