Sort ASC by a field should give results where that field is not present

[adityaverma21]

Describe` the bug
Sort ASC by a field should order results where that field is not present. But currently sorting by both ASC or DESC gives documents where that field is not present at the last.

To Reproduce
Steps to reproduce the behavior:

1. Schema Definition

schema test {
  document test {
      field name type string {
         indexing: summary | index
      }
      field lastdate type array<long> {
         indexing: summary | attribute
      }
      field title type array<string> {
         indexing: summary | attribute
      }
  }
}

2. Documents:
   doc1

{
  "fields": {
     "name": "aditya",
    "lastdate": [1693560729001],
     "title" : ["verma"]
    }
}

doc2

{
  "fields": {
     "name": "aditya",
    "lastdate": [1693560729050],
     "title" : ["adi"]
    }
}

doc3

{
  "fields": {
     "name": "aditya",
    "lastdate": []
    }
}

3. Query:
   select * from sources * where name contains "aditya" order by title ASC limit 10 offset 0;
   This returns the docs in order doc2 -> doc1 -> doc3

Expected behavior
The behaviour should be that docs where the field is not present should be ordered first in ascending order.
doc3 -> doc2 -> doc1

Environment (please complete the following information):

• RHEL8
• Podman

Vespa version
8.221.29

[bjormel]

Duplicate of #31106

If the field is unset for a document, the sort value for that document will be lower than the value for any other document with a value set.

If you need another behaviour you can use a document processor to set a value when it is missing when ingesting data.
See Fields

A field can not be defined with a default value. Use a document processor to assign a default to document put/update operations.

[baldersheim]

This is not the same as #31106. This relates to multi value fields, field title type array<string>.
As a short string sorts before a longer one, all else being equal, you can argue the same here.

[toregge]

Issue #26681 contained a request for supporting sorting on multivalue attributes.

From reference doc (https://docs.vespa.ai/en/reference/sorting.html#multivalue-sort-attribute):

When sorting on a multivalue attribute (array or weightedset) one of the values for the document is selected to be used for sorting. Ascending sort order uses the lowest value while descending sort order uses the highest value. A document without any values is considered worse than a document with values, regardless of sort order.

doc3 doesn't have any values for title, thus it is considered worse than doc1 and doc2 regardless of sort order.

[adityaverma21]

Can i raise a request for feature where sorting on multivalued attributes is consistent with single valued attributes i.e for fields where value is not set, those documents should be ranked first in ASC order even if its multivalued attributes.