message: Issue QUANTILE_DISC for VARCHAR
  Add some boundary tests.
diff: @@ -70,14 +70,47 @@ FROM range(0,100) tbl(d)
  endloop
  +#
  # VARCHAR. Remember, this is dictionary ordering, not numeric ordering!
  +#
  query III
  SELECT quantile_disc(d::VARCHAR, 0.1), quantile_disc(d::VARCHAR, 0.5), quantile_disc(d::VARCHAR, 0.9)
  FROM range(0,100) tbl(d)
  ----
  17 53 9
  -# multiple groups
  +# A string column that has only NULL values
  +query I
  +SELECT quantile_disc(NULL::VARCHAR, 0.1)
  +FROM range(0,100) tbl(d)
  +----
  +NULL
  +
  +# A string column that has long strings (>12 characters)
  +query I
  +SELECT quantile_disc('prefix-' || d::VARCHAR || '-suffix', 0.1)
  +FROM range(0,100) tbl(d)
  +----
  +prefix-17-suffix
  +
  +# Using median itself on a string (just an alias for quantile_disc)
  +query I
  +SELECT median(d::VARCHAR)
  +FROM range(0,100) tbl(d)
  +----
  +53
  +
  +# Median on an empty table
  +query I
  +SELECT median(d::VARCHAR)
  +FROM range(0,100) tbl(d)
  +WHERE d > 100
  +----
  +NULL
  +
  +#
  +# Multiple groups
  +#
  query II
  SELECT mod(r,10) as g, quantile_disc(r, 0.1) FROM quantile GROUP BY 1 ORDER BY 1
  ----
  