@Override
    public String getKeyColumn() {
        String column = getColumn(KEY_COLUMN_TUPLE_INDEX);
        if (column == null) {
            // fallback to global map key config
            return this.registry.get(MapEntryMappers.class).getKeyColumn();
        }
        return column;
    }
Mappers similar to {@link org.jdbi.v3.core.mapper.MapEntryMappers} but map entries in vavr are in fact
of type {@link io.vavr.Tuple2}.
vavr/src/main/java/org/jdbi/v3/vavr/TupleMappers.java