Incomplete Optimization A properly designed relational system decomposes a SQL command to a lower level algebraic form. It then uses mathematical theorems to transform the request into optimized form for processing. Other systems don't go through this step and must use ad-hoc optimization techniques. This results in incomplete optimization. Inadequate optimizations force users to do extensive tuning to get reasonable performance from the systems.
Simliar queries can have wide variations in execution timing.
Incomplete optimization produces uneven performance. Uneven Performance One of the most noticeable aspects of ad-hoc relational implementations is uneven performance. Because these systems perform inadequate transforms during optimization, equivalent queries can have wide variations in performance. Pascal constructs 7 queries using different formulations but producing the same result set on an example database. Ingres timings were basically the same for all 7 queries.