ORM performance cost

Question

Does anyone have any experience that indicates what kind of performance hit a developer could expect by choosing to use an ORM (in Django, RoR, SQLAlechemy, etc) over SQL and hand-designed databases? I imagine there are complicating issues, including whether specifying a database within the constraints of an ORM increases or decreases the chances of creating an efficient database structure (based on the developer's level of experience), and the question of how well the developer constructs either the SQL or ORM-based queries (again based on his/her experience). Any information regarding these or intrinsic performance issues would be really interesting to me.

Answer 1

My advice is not to worry about this until you need to - don't optimise prematurely. An ORM can provide many benefits to development speed, code readability and can remove a lot of code repetition. I would recommend using one if it will make your application easier to develop.

As you progress through the development use benchmarks and profiling to determine the bottlenecks in the code and if needed you can bypass the ORM and use manual queries where they are required. Normally you will be able to improve the speed of the ORM using caching and database indexes (amongst other things) and then you can decide where manual queries are required. For the most part, the ORM performance will probably acceptable and the benefits of using it will far outweigh the performance cost.