public Set<SourceName> sourcesWithField(
      final Optional<SourceName> source,
      final ColumnName target
  ) {
    if (!source.isPresent()) {
      return sourceSchemas.entrySet().stream()
          .filter(e -> e.getValue().findColumn(target).isPresent())
          .map(Entry::getKey)
          .collect(Collectors.toSet());
    }

    final SourceName sourceName = source.get();
    final LogicalSchema sourceSchema = sourceSchemas.get(sourceName);
    if (sourceSchema == null) {
      return ImmutableSet.of();
    }

    return sourceSchema.findColumn(target).isPresent()
        ? ImmutableSet.of(sourceName)
        : ImmutableSet.of();
  }
@Test
  public void shouldFindNoQualifiedField() {
    assertThat(sourceSchemas.sourcesWithField(Optional.of(ALIAS_1), V2), is(empty()));
  }