presto-accumulo/src/main/java/io/prestosql/plugin/accumulo/index/Indexer.java/Indexer/getMetricIterators
class Indexer:    public static Collection<IteratorSetting> getMetricIterators(AccumuloTable table)
    {
        String cardQualifier = new String(CARDINALITY_CQ, UTF_8);
        String rowsFamily = new String(METRICS_TABLE_ROWS_CF.array(), UTF_8);

        // Build a string for all columns where the summing combiner should be applied,
        // i.e. all indexed columns
        StringBuilder cardBuilder = new StringBuilder(rowsFamily + ":" + cardQualifier + ",");
        for (String s : getLocalityGroups(table).keySet()) {
            cardBuilder.append(s).append(":").append(cardQualifier).append(',');
        }
        cardBuilder.deleteCharAt(cardBuilder.length() - 1);

        // Configuration rows for the Min/Max combiners
        String firstRowColumn = rowsFamily + ":" + new String(METRICS_TABLE_FIRST_ROW_CQ.array(), UTF_8);
        String lastRowColumn = rowsFamily + ":" + new String(METRICS_TABLE_LAST_ROW_CQ.array(), UTF_8);

        // Summing combiner for cardinality columns
        IteratorSetting s1 = new IteratorSetting(1, SummingCombiner.class, ImmutableMap.of("columns", cardBuilder.toString(), "type", "STRING"));

        // Min/Max combiner for the first/last rows of the table
        IteratorSetting s2 = new IteratorSetting(2, MinByteArrayCombiner.class, ImmutableMap.of("columns", firstRowColumn));
        IteratorSetting s3 = new IteratorSetting(3, MaxByteArrayCombiner.class, ImmutableMap.of("columns", lastRowColumn));

        return ImmutableList.of(s1, s2, s3);
    }