Fix extra solution per entity in optional property branches

[ddeboer]

Follow-up to #183.

That PR starts the UNION of optional properties with an empty group, which keeps the solution for an entity that has none of them. The empty group also adds a solution for every entity that does have them, at each level of a nested schema. A store that does not deduplicate CONSTRUCT output, such as QLever, sends the triples of each of those solutions again.

With this change the UNION sits in an OPTIONAL instead:

?iri x:requiredString ?iri_0 .
OPTIONAL {
  { ?iri x:optionalArray ?iri_1 . }
  UNION { ?iri x:otherOptionalArray ?iri_2 . }
}

An entity with 3 values for one optional array and 3 for another now comes back as 3 + 3 = 6 solutions, where 2.10.0 gives 1 + 3 + 3 = 7. An entity with none still comes back as 1. A level with one optional property gets the same shape, which amounts to the plain OPTIONAL it had before #183.

Measurements

I ran findByIri() against the QLever endpoint of the Dataset Knowledge Graph, on its two largest VoID descriptions, with two schemas:

• class partitions: three nested levels, with one, one and three optional arrays;
• summary: one level with six optional values and three optional arrays.

Response size:

	2.7.2	2.10.0	this change
class partitions, som-persons-in-music	9968 KB	14792 KB	9984 KB
class partitions, iisg-kg	8849 KB	12488 KB	9460 KB
summary, som-persons-in-music	8736 KB	389 KB	388 KB
summary, iisg-kg	8280 KB	324 KB	324 KB

The last column is the 2.10.0 npm build with this change applied to it by hand, not a build of this branch. I compared the decoded entities whole, ignoring array order: they are identical in all three columns.

On the nested schema 2.10.0 was 48% and 41% larger than 2.7.2. iisg-kg stays 7% above 2.7.2: where two optional properties on one level each have a single value, their sum (2 solutions) is higher than their product (1).

An alternative that does not work

I first made only the first branch optional: { OPTIONAL { … } } UNION { … }. That group is evaluated without the bindings of ?iri, so an entity with none of the optional properties is lost. On iisg-kg that dropped 1085 of the 3148 property partitions: the ones without a nested partition.

Tests

The quad count in “Should not multiply the values of optional properties” goes from 34 to 28.