A classical approach to study the complexity of a problem $P$ is to efficiently reduce a well known problem $P'$ to $P$, thus showing that $P$ is at least as difficult as $P'$. The TCS literature contains thousands of such reductions.
This induces a directed graph over problems: $P' \longrightarrow P$ if $P'$ was reduced to $P$.
First question: Is there a way to collect (large parts of) this graph? Maybe web sites or publications listing many reductions in a suitable way?
This graph may contain interesting information on how we work, the importance of some problems, and maybe on problems themselves. For instance, problems with a very large in-degree are very often used for reductions. Cycles indicate proofs of complexity completeness. The sets of problems reduced to a same problem (the in-neighbors of this problem) may also be interesting. The out-degree of problems will be low in general, but not always, and this also brings information. The transitive links in this graph are also interesting, although trivial since reduction is transitive.
Second question: What information shall we obtain from this graph, in your opinion?
Notice that I am talking of the graph of reductions that have been proved in the literature, not all possible reductions, which would lead to a much simpler graph because of transitivity.