What indexing strategies improve database performance?

-- Composite: serves WHERE (tenant_id) and (tenant_id, created_at),
-- but NOT (created_at) alone -> the leftmost-prefix rule.
CREATE INDEX idx_events_tenant_time ON events (tenant_id, created_at);

-- Covering: INCLUDE the selected columns so the query is index-only
-- (no extra trip to the table heap).
CREATE INDEX idx_orders_cover ON orders (user_id) INCLUDE (status, total_cents);

-- Partial: index only the rows you actually query -> smaller and faster.
CREATE INDEX idx_orders_open ON orders (user_id) WHERE status = 'open';