-- One question per symbol per canonical (older, newer) session-half slot pair.

ALTER TABLE market_history_prospective_questions
  DROP CONSTRAINT IF EXISTS market_history_prospective_questions_compare_until_symbol_key;

ALTER TABLE market_history_prospective_questions
  DROP CONSTRAINT IF EXISTS market_history_prospective_questions_slot_pair_symbol_key;

ALTER TABLE market_history_prospective_questions
  ADD CONSTRAINT market_history_prospective_questions_slot_pair_symbol_key
  UNIQUE (symbol, older_slot_start, newer_slot_start);

CREATE INDEX IF NOT EXISTS market_history_prospective_questions_slot_pair_idx
  ON market_history_prospective_questions (older_slot_start, newer_slot_start);