presto-ml/src/main/java/io/prestosql/plugin/ml/LearnStateFactory.java/GroupedLearnState/getFeatureVectors
class GroupedLearnState:        @Override
        public List<FeatureVector> getFeatureVectors()
        {
            List<FeatureVector> featureVectors = featureVectorsArray.get(groupId);
            if (featureVectors == null) {
                featureVectors = new ArrayList<>();
                size += ARRAY_LIST_SIZE;
                featureVectorsArray.set(groupId, featureVectors);
            }
            return featureVectors;
        }