Search: ElasticSearch vs Postrsql
Postresql: A basic database search for the database that stores all your MP data.
Elasticsearch: An open-source, broadly-distributable, readily-scalable, enterprise-grade search engine. Accessible through an extensive and elaborate API, Elasticsearch can power extremely fast searches that support your data discovery applications.
Here is an article that provides a bit more information on ElasticSearch:
Default Search Order
If the search is Geo based, then the listings are sorted by closeness (ascending) to the target location. Otherwise, they are sorted by a score generated by elasticsearch for each result item.
Admin Dash > Manage > Transactable Types > Search > 'Search Engine'