Queries are compiled, i.e., a query plan is created and then executed. Depending on various things that plan may get re-used the next time the query is run. So

Queries are compiled, i.e., a query plan is created and then executed. Depending on various things that plan may get re-used the next time the query is run. So what you are talking about exists. Databases can examine the plans, database statistics, and the resulting metrics from executing the plan and make recommendations about indexes.