presto-main/src/main/java/io/prestosql/operator/index/IndexSnapshot.java/IndexSnapshot/IndexSnapshot
class IndexSnapshot:    public IndexSnapshot(LookupSource values, LookupSource missingKeys)
    {
        this.values = requireNonNull(values, "values is null");
        this.missingKeys = requireNonNull(missingKeys, "missingKeys is null");
    }