presto-raptor-legacy/src/main/java/io/prestosql/plugin/raptor/legacy/metadata/ShardIterator.java/ShardIterator/computeNext
class ShardIterator:    @Override
    protected BucketShards computeNext()
    {
        try {
            return merged ? computeMerged() : compute();
        }
        catch (SQLException e) {
            throw metadataError(e);
        }
    }