SQL: an index on a low-cardinality column is usually ignored

CREATE INDEX idx_orders_status ON orders (status);   -- 3 distinct values

SELECT * FROM orders WHERE status = 'PAID';       -- 73% of rows -> seq scan
SELECT * FROM orders WHERE status = 'REFUND';     -- 1% of rows  -> index scan

-- A partial index only covers the selective value, and is far smaller:
CREATE INDEX idx_orders_refund ON orders (created_at)
  WHERE status = 'REFUND';

status='PAID'   -> Seq Scan   (rows=7,210 of 9,876)
status='REFUND' -> Index Scan (rows=118)

-- The same index, two plans, both correct. "The index is not being used"
-- is usually the planner being right.
--
-- The partial index above is 118 entries instead of 9,876.