Drupal 8: Override service methods(service decorator)

gurei.services.yml

  workspaces.entity.query.sql:
    decorates: entity.query.sql
    class: Drupal\workspaces\EntityQuery\QueryFactory
    arguments: 
      - '@database'
      - '@workspaces.manager'
    public: false
    decoration_priority: 50
    tags:
      - { name: backend_overridable }

QueryFactory.php

<?php

/**
 * Workspaces-specific entity query implementation.
 */
class QueryFactory extends BaseQueryFactory {

  /**
   * {@inheritdoc}
   */
  public function getAggregate(EntityTypeInterface $entity_type, $conjunction) {
    $class = QueryBase::getClass($this->namespaces, 'QueryAggregate');
    return new $class($entity_type, $conjunction, $this->connection, $this->namespaces, $this->workspaceManager);
  }

}