Benchmarking PostgreSQL's SELECT Query Planning and Performance on Columns Aggregates
Executing a single query doesn’t always literally mean executing exactly ONE query. Sometimes PostgreSQL (like any other relation database) will have to extract parts of your query (subqueries) and re-execute it as many times as needed to generate the results you are expecting it to generate. This article shows how N+1 issues can also happen without an explicit intent to produce them. ...