Most databases have ways for you to specify the exact plan you want used already. Since you mentioned SQL Server, you can force join order, force index usage, f

Most databases have ways for you to specify the exact plan you want used already. Since you mentioned SQL Server, you can force join order, force index usage, force the type of join used (loop, merge, hash), etc. There are terrible drawbacks using any of those techniques (data changes, indexes being modified / dropped, etc) which is why, imo, they should very rarely be used. However, 'allowing direct specification of the query plan' is already implemented in that respect. The better way to address the problem of different parameters causing performance issues is forcing recompiles which will solve the problem 95% of the time.