Sort by relevancy of matching terms

Did you debug the query?

I think we create the query as "q" and only conditions as "fq". So I think that it is correct that conditions should not influence the scoring as all documents have to meet the condition.

Which use-case does create filter queries should influence the scoring?

Thanks for the answer. Yes, I did debug the query (attached) and the conditions from facets turn out to really be living in fq.

What did the job for me was the following:

namespace Drupal\custom\EventSubscriber;

use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\search_api_solr\Event\PostConvertedQueryEvent;
use Drupal\search_api_solr\Event\SearchApiSolrEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Event subscriber for solr queries.
 */
final class CustomSolrQuerySubscriber implements EventSubscriberInterface {

  /**
   * Reacts to the post convert query event.
   *
   * @param \Drupal\search_api_solr\Event\PostConvertedQueryEvent $event
   *   The post converted query event.
   */
  public function solrQueryAlter(PostConvertedQueryEvent $event) {
    $search_api_query = $event->getSearchApiQuery();
    // The "content_relevancy_sorting" tag is set from Views UI.
    if (in_array('content_relevancy_sorting', $search_api_query->getTags(), TRUE)) {
      $sorts = &$search_api_query->getSorts();
      if (isset($sorts['search_api_relevance'])) {
        $solarium_query = $event->getSolariumQuery();
        $filter_queries = $solarium_query->getFilterQueries();
        $q = [];
        foreach ($filter_queries as $filter_id => $filter_query) {
          if (!str_starts_with($filter_id, 'filters_')) {
            continue;
          }
          $q[] = $filter_query->getOption('query');
        }
        $string_query = $solarium_query->getQuery() . ' AND ' . implode(' AND ', $q);
        $solarium_query->setQuery($string_query);
      }
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents(): array {
    return [
      SearchApiSolrEvents::POST_CONVERT_QUERY => 'solrQueryAlter',
    ];
  }

}

What I basically do in the query above is I take the conditions from fq and also add them with "AND" to the "q" parameter. But I wonder whether this should really require extra code instead of UI.

I still don't understand the use-case.
All result items must match any of the filters. How should a result item match more filters than another?

Could you describe a concrete example?

Yes, let me explain to you my exact use case... Intersection!

Let's say we have two nodes:

Node #1:
- Tags: A, B, C, D

Node #2:
- Tags: C, D, E

A simple user uses facets to filter the results of an indexed view displaying the nodes matching B or C or D.
As an admin I set the view to sort the results based on Solr's score. So, I expect Solr to return Node #1 first and then Node #2 since #1 matches more terms than #2 - based on what the user has filtered by.

Since both nodes match the B or C or D condition, both are displayed. But... since using fq instead of q, their relevancy score is "1.0" for both.

Thanks for the explanation.

But this is how it works. Facets are filters and don't influence the scoring.
Converting them into a query will break a lot of different facets features, especially tagging and excluding:
https://solr.apache.org/guide/solr/latest/query-guide/faceting.html#tagg...

I understand your point. In case I come up with another idea in the future, a more generic approach let's say, I will come back here.

In the meantime, I take the opportunity to change this to "Won't fix" in order to help anyone coming here in the future, considering it fixed and looking for what was changed.