code: public static void updateSizeEstimates(String keyspace, String table, Map<Range<Token>, Pair<Long, Long>> estimates)
      {
          long timestamp = FBUtilities.timestampMicros();
          CFMetaData estimatesTable = CFMetaData.SizeEstimatesCf;
          Mutation mutation = new Mutation(Keyspace.SYSTEM_KS, UTF8Type.instance.decompose(keyspace));
  
          // delete all previous values with a single range tombstone.
          mutation.deleteRange(SIZE_ESTIMATES_CF,
                               estimatesTable.comparator.make(table).start(),
                               estimatesTable.comparator.make(table).end(),
                               timestamp - 1);
  
          // add a CQL row for each primary token range.
          ColumnFamily cells = mutation.addOrGet(estimatesTable);
          for (Map.Entry<Range<Token>, Pair<Long, Long>> entry : estimates.entrySet())
          {
              Range<Token> range = entry.getKey();
              Pair<Long, Long> values = entry.getValue();
              Composite prefix = estimatesTable.comparator.make(table, range.left.toString(), range.right.toString());
              CFRowAdder adder = new CFRowAdder(cells, prefix, timestamp);
              adder.add("partitions_count", values.left)
                   .add("mean_partition_size", values.right);
          }
  
          mutation.apply();
      }
docs: Writes the current partition count and size estimates into SIZE_ESTIMATES_CF