presto-main/src/main/java/io/prestosql/operator/aggregation/QuantileDigestAggregationFunction.java/QuantileDigestAggregationFunction/getOrCreateQuantileDigest
class QuantileDigestAggregationFunction:    private static QuantileDigest getOrCreateQuantileDigest(QuantileDigestState state, double accuracy)
    {
        QuantileDigest qdigest = state.getQuantileDigest();
        if (qdigest == null) {
            qdigest = new QuantileDigest(accuracy);
            state.setQuantileDigest(qdigest);
            state.addMemoryUsage(qdigest.estimatedInMemorySizeInBytes());
        }
        return qdigest;
    }