[bunderbunder]

It's a great heuristic.

A big part of why it works out that way is that the query planner can only optimize for the query you give it. If you give it a bunch of small queries, it can only make relatively inconsequential micro-optimizations. The one big query gives it a lot more degrees of freedom and opportunities to make big gains.

Here's another great resource for getting more out of relational databases: https://use-the-index-luke.com/