When is optimisation premature?

Question

As Knuth said,

We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.

Answer 1

I try to only optimise when a performance issue is confirmed.

My definition of premature optimisation is 'effort wasted on code that is not known to be a performance problem.' There is most definitely a time and place for optimisation. However, the trick is to spend the extra cost only where it counts to the performance of the application and where the additional cost outweighs the performance hit.

When writing code (or a DB query) I strive to write 'efficient' code (i.e. code that performs its intended function, quickly and completely with simplest logic reasonable.) Note that 'efficient' code is not necessarily the same as 'optimised' code. Optimisations often introduce additional complexity into code which increases both the development and maintenance cost of that code.

My advice: Try to only pay the cost of optimisation when you can quantify the benefit.