max_children exist only in top level nested sort

[erayarslan]

FIX
related with (#33587 (comment))

If we have query like that;

GET /products/_search
{
  "sort": [
    {
      "variants.listings.price": {
        "order": "desc",
        "nested": {
          "path": "variants",
          "nested": {
            "max_children": 1,
            "path": "variants.listings"
          }
        }
      }
    }
  ]
}

Elastic going to ignore max_children=1, cause we passed top level nested sort object in here (example flow);

elasticsearch/server/src/main/java/org/elasticsearch/search/sort/FieldSortBuilder.java

Lines 334 to 340 in 3f18cf7

	if (nestedSort != null) {
	if (nestedSort.getNestedSort() != null && nestedSort.getMaxChildren() != Integer.MAX_VALUE) {
	throw new QueryShardException(context,
	"max_children is only supported on last level of nested sort");
	}
	nested = resolveNested(context, nestedSort);
	} else {

elasticsearch/server/src/main/java/org/elasticsearch/search/sort/SortBuilder.java

Lines 173 to 186 in 3f18cf7

	protected static Nested resolveNested(QueryShardContext context, NestedSortBuilder nestedSort) throws IOException {
	final Query childQuery = resolveNestedQuery(context, nestedSort, null);
	if (childQuery == null) {
	return null;
	}
	final ObjectMapper objectMapper = context.nestedScope().getObjectMapper();
	final Query parentQuery;
	if (objectMapper == null) {
	parentQuery = Queries.newNonNestedFilter();
	} else {
	parentQuery = objectMapper.nestedTypeFilter();
	}
	return new Nested(context.bitsetFilter(parentQuery), childQuery, nestedSort, context.searcher());
	}

elasticsearch/server/src/main/java/org/elasticsearch/index/fielddata/fieldcomparator/FloatValuesComparatorSource.java

Lines 69 to 74 in 3f18cf7

	} else {
	final BitSet rootDocs = nested.rootDocs(context);
	final DocIdSetIterator innerDocs = nested.innerDocs(context);
	final int maxChildren = nested.getNestedSort() != null ? nested.getNestedSort().getMaxChildren() : Integer.MAX_VALUE;
	selectedValues = sortMode.select(values, dMissingValue, rootDocs, innerDocs, context.reader().maxDoc(), maxChildren);
	}

So i think we must use max_children on top level nested sort or we need to pass last level nested sort.

In this PR, I allow to max_children on top level nested sort.

[elasticmachine]

Pinging @elastic/es-search

[jimczi]

Good catch @erayarslan , I left a minor comment but the change looks good.

[erayarslan]

Thanks @jimczi , I updated integration test.

Can you please take a look?

[jimczi]

@elasticmachine test this please

[erayarslan]

CI failed. I think my branch outdated. Now, its okay.
Can you trigger CI again? @jimczi

[jimczi]

@elasticmachine test this please

[jimczi]

LGTM, thanks @erayarslan

[erayarslan]

thank you for your time @jimczi