What is expansion revenue rate?

-- subscription_changes has one row per MRR delta with a change_type.
-- Expansion and contraction are reported side by side, never netted.
WITH months AS (
  SELECT generate_series(
    date_trunc('month', CURRENT_DATE - INTERVAL '12 months'),
    date_trunc('month', CURRENT_DATE - INTERVAL '1 month'),
    INTERVAL '1 month'
  )::date AS month
), starting AS (
  -- Snapshot written on the first of each month; the denominator
  -- must not drift as the month plays out.
  SELECT snapshot_month AS month, SUM(mrr_usd) AS starting_mrr
  FROM mrr_snapshots
  GROUP BY 1
), changes AS (
  SELECT
    date_trunc('month', effective_at)::date AS month,
    SUM(mrr_delta_usd) FILTER (
      WHERE change_type IN ('upsell', 'cross_sell', 'seat_expansion')
    ) AS expansion_mrr,
    SUM(-mrr_delta_usd) FILTER (
      WHERE change_type IN ('downgrade', 'seat_reduction')
    ) AS contraction_mrr,
    COUNT(DISTINCT account_id) FILTER (
      WHERE change_type IN ('upsell', 'cross_sell', 'seat_expansion')
    ) AS expanding_accounts
  FROM subscription_changes
  GROUP BY 1
)
SELECT
  m.month,
  ROUND(st.starting_mrr, 2) AS starting_mrr,
  ROUND(COALESCE(c.expansion_mrr, 0), 2) AS expansion_mrr,
  ROUND(COALESCE(c.contraction_mrr, 0), 2) AS contraction_mrr,
  ROUND(
    100.0 * COALESCE(c.expansion_mrr, 0)
    / NULLIF(st.starting_mrr, 0), 2
  ) AS expansion_rate_pct,
  ROUND(
    100.0 * COALESCE(c.contraction_mrr, 0)
    / NULLIF(st.starting_mrr, 0), 2
  ) AS contraction_rate_pct,
  COALESCE(c.expanding_accounts, 0) AS expanding_accounts
FROM months m
JOIN starting st USING (month)
LEFT JOIN changes c USING (month)
ORDER BY m.month;