message: Issue Implement MAD
  Add ReplaceIndex coverage.
diff: @@ -209,3 +209,21 @@ NULL NULL 5.000000
  17 NULL 5.000000
  18 18 5.000000
  19 NULL 5.000000
  +
  +#
  +# ReplaceIndex coverage
  +#
  +statement ok
  +CREATE TABLE coverage AS SELECT * FROM (VALUES
  + (1), (2), (3), (1)
  + ) tbl(r)
  +
  +query II
  +SELECT r, mad(r) OVER (ORDER BY r ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
  +FROM coverage
  +ORDER BY 1
  +----
  +1 0.000000
  +1 0.000000
  +2 1.000000
  +3 0.500000
  