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Your Optimizer Found a Good Answer—Is It the Only One?

One reported optimizer solution does not prove uniqueness. Learn how tied optima differ from near-optimal answers and which methods can reveal or select alternatives.
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No—not necessarily. An optimizer usually reports one solution, but other distinct decisions may achieve the same optimal objective value. The printed answer alone does not prove that it is unique. It is also important to distinguish a tied optimum from a solution that is merely close to the best value.

What does “the only one” mean?

There are three different things to keep separate:

  • The optimal value: the best objective value the model can achieve.
  • An optimal solution: a feasible set of decisions that achieves that value.
  • A near-optimal solution: a feasible set of decisions whose objective is close to, but potentially worse than, the best value.

Different decision assignments can tie for the same optimum. In some linear programs, the optimum is not just a handful of separate answers: every point along a line segment between optimal points can also be optimal. Gurobi’s objective documentation notes that optimization problems can have multiple optimal solutions as well as many solutions within a small objective gap of the optimum.

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Why can different answers be equally good?

The solver optimizes the objective and constraints you gave it; it cannot infer preferences the model does not express. If a scheduling model minimizes the number of workers but does not prefer particular workers, interchangeable assignments may have the same objective value. Gurobi illustrates this kind of issue in its discussion of optimization geometry and instability.

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That means “best” is relative to the model. If fairness, robustness, convenience, or another real-world consideration matters, it needs to be represented in the model or used as an explicit tie-breaker. Inspect the decision variables, constraints, objective, and any secondary criteria before interpreting one returned assignment as uniquely best in practice.

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Does a solver’s “optimal” result prove uniqueness?

No. A standard solve may return a single optimal solution without determining whether others exist. Gurobi states that it returns one optimal solution for continuous models. In mixed-integer models, solutions retained during an ordinary run should not be treated as a systematic enumeration of alternatives.

Also, “optimal” is a numerical claim made under the solver’s tolerances and stopping rules. Floating-point calculations can make exact equality difficult to assess, and an accepted optimality gap may mean a reported result is close to the proven best value rather than mathematically identical to it. Conversely, objective values that display the same rounded digits may differ at greater precision. Google’s OR-Tools LP guidance discusses both tolerances and non-unique solutions. Do not infer uniqueness from a single printed answer or from equality at the displayed precision.

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How can you look for alternatives?

Approach What it can tell you Important limitation
Use the one solution from a standard solve Provides an answer when any optimum is sufficient. Does not reveal whether another optimum exists.
Configure a mixed-integer solution pool Can seek or retain additional discrete solutions; the pool settings determine what alternatives are sought. A default pool is not an exhaustive catalogue. Search for the requested best solutions only has that meaning if the systematic search completes. This feature does not provide multiple optimal LP solutions.
Analyze alternative optima in a linear program Addresses whether the continuous model has other points on its optimal face. A MIP solution pool is not the method for enumerating multiple optimal LP points.
Add a secondary or hierarchical objective Selects among primary-optimal choices according to a preference you specify. You must define how strictly the primary objective is preserved and any permitted degradation.

Gurobi’s solution-pool documentation describes the available mixed-integer search modes. Its help article, “How do I find additional solutions to a model?”, also explains that the MIP pool does not return multiple optimal solutions for an LP.

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How should you choose among tied answers?

First decide what “better” means beyond the primary objective. If two schedules cost the same, for example, you might prefer one that distributes shifts more fairly. Encode that preference as a secondary objective, or use a hierarchical objective that ranks goals by priority. Gurobi’s multiple-objective documentation describes hierarchical optimization. When using it, make the allowed degradation in higher-priority objectives explicit; otherwise, the tie-breaker may weaken the goal you intended to protect.

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