The problem is NP-complete (proven) for all input data (without exception).
We assume that P != NP.
Is it possible that there is an (infinitely large) subset of the problem, for which this subset is in P?
Theoretical question.
The problem is NP-complete (proven) for all input data (without exception).
We assume that P != NP.
Is it possible that there is an (infinitely large) subset of the problem, for which this subset is in P?
Theoretical question.
Your question doesn't make sense:
The problem is NP-complete (proven) for all input data (without exception).
This is not a thing. NP-completeness is a property of entire sets, not of specific inputs. It's fairly trivial to show that, if you choose a specific input, any problem is $O(1)$ on that input: you just output yes or no, depending which is correct for your input. When you do this, all of algorithm analysis breaks down and everything ends up useless. So, we don't do this.
$P$, $NP$, $NP$-completeness, polynomial time, etc. are all related to asymptotic complexity: as the size of input grows, how does the run-time grow? This only makes sense when you look across different input, the same way the derivative of a point in calculus is a property that takes into account the curve surrounding the point.
We assume that P != NP.
Again, this doesn't affect your answer. If $P = NP$, then every $P$ set is $NP$-complete,* so there are a bunch of subsets in $P$. And if $P=NP$, the examples that people have given are all valid.
Is it possible that there is a subset of the problem (infinitely large), that this problem is in P?
Yes, and the other answers have given great examples of this. But for posterity, here's yet another counter example.
* Except for $\emptyset$ and $\Sigma^*$, which aren't $NP$-complete for technical reasons relating to the form of reductions we use to define completeness.
In fact, you don't need the P$\,\neq\,$NP hypothesis, since there are even infinite constant-time decidable subsets of NP-complete problems. For any NP-complete language $L\subseteq\{0,1\}^*$, let $L' = \{0w\mid w\in L\}\cup\{1w\mid w\in\{0,1\}^*\}$. $L'$ is still NP-complete (trivial reduction from $L$), but it contains the infinite constant-time decidable subset of all strings beginning with $1$.
As I understand your question, yes, this is possible. As an example, consider the (infinitely large) subset of SAT that contains all satisfiable formulae with no negations. This subset is trivially in P.
Take the example of $\mathsf{3\text{-}Coloring}$ problem, it is $\mathsf{NP\text{-}Complete}$ in general, but for trees (there are infinitely many ) it is in $\mathsf{P\text{}}$
Graph coloring, maximum clique and maximum independent set are $\mathcal{NP}$-complete (decision versions), but polynomial on perfect graphs.
Take the Travelling Salesman problem: Given are n cities, and the distances between each pair of cities, and a limit d. Is there a route touching each city once and then returning to the starting point with a total length ≤ d?
For any set of cities and distances, for small values of d ("small" depending on the distances) it will be easy to show that there is no solution, and for large values of d it is easy to find a solution. So you can find a subset of the problem where d is large or small enough where you can solve any instance in polynomial time.