presto-main/src/main/java/io/prestosql/operator/HashBuilderOperator.java/HashBuilderOperator/disposeLookupSourceIfRequested
class HashBuilderOperator:    private void disposeLookupSourceIfRequested()
    {
        checkState(state == State.LOOKUP_SOURCE_BUILT);
        verify(lookupSourceNotNeeded.isPresent());
        if (!lookupSourceNotNeeded.get().isDone()) {
            return;
        }

        index.clear();
        localRevocableMemoryContext.setBytes(0);
        localUserMemoryContext.setBytes(index.getEstimatedSize().toBytes());
        lookupSourceSupplier = null;
        close();
    }