At what point does knowledge of Query Planner / Query Optimizer become critical path?

Question

For simple queries on small DBs (say <10 parameters and <1M records), it may not be necessary to know how a query executes... but at some point performance and resource constraints kick in.

Has anyone found some good rule of thumb type guidelines to indicate when a query design needs to be re-evaluted?

I'm aware full answer varies by DB technology and resources. Given the audience of learners here, something like... "Here's what's practical on a phone vs a laptop vs an EC2 Micro" may be a good start.

Answer 1

Query Performance is not mainly affected by number of rows. It is most affected by poorly written SQL statements, poor DB design and missing indexes all of which increase the overall cost of query