presto-main/src/main/java/io/prestosql/operator/index/IndexSourceOperator.java/IndexSourceOperator/addSplit
class IndexSourceOperator:    @Override
    public Supplier<Optional<UpdatablePageSource>> addSplit(Split split)
    {
        requireNonNull(split, "split is null");
        checkState(source == null, "Index source split already set");

        IndexSplit indexSplit = (IndexSplit) split.getConnectorSplit();

        // Normalize the incoming RecordSet to something that can be consumed by the index
        RecordSet normalizedRecordSet = probeKeyNormalizer.apply(indexSplit.getKeyRecordSet());
        ConnectorPageSource result = index.lookup(normalizedRecordSet);
        source = new PageSourceOperator(result, operatorContext);

        Object splitInfo = split.getInfo();
        if (splitInfo != null) {
            operatorContext.setInfoSupplier(() -> new SplitOperatorInfo(splitInfo));
        }

        return Optional::empty;
    }