def get_random_variant(df, seed=0):
  """
  returns a random chromosome, start position and sampleId for querying
  """
  row = df.sample(False, 0.1, seed=seed).limit(1).collect()
  chrom = row[0].contigName
  start = row[0].start
  sampleId = row[0].sampleId
  return chrom, start, sampleId