[DF] Remove call to VecOps::Combinations in ODB query 6

VecOps::Combinations creates a jets.size()x3 matrix.
Moreover, this newly created matrix is used only as index, such as
jets[c[0][i]].
It makes sense to get rid of this matrix and access the elements
directly. In this case, we have contiguous memory access only.
Hence, improved performance of the find_triject function.

root/tree/dataframe/RDataFrameOpenDataBenchmarks.cxx

@@ -297,24 +297,29 @@ BENCHMARK(BM_RDataFrame_OpenDataBenchmark5_compiled)->Unit(benchmark::kMilliseco
 
 ROOT::RVec<std::size_t> benchmark6_find_trijet(Vec<XYZTVector> jets)
 {
-   const auto c = ROOT::VecOps::Combinations(jets, 3);
-
+   constexpr std::size_t n = 3;
    float distance = 1e9;
    const auto top_mass = 172.5;
-   std::size_t idx = 0;
-   for (auto i = 0u; i < c[0].size(); i++) {
-      auto p1 = jets[c[0][i]];
-      auto p2 = jets[c[1][i]];
-      auto p3 = jets[c[2][i]];
-      const auto tmp_mass = (p1 + p2 + p3).mass();
-      const auto tmp_distance = std::abs(tmp_mass - top_mass);
-      if (tmp_distance < distance) {
-         distance = tmp_distance;
-         idx = i;
+   std::size_t idx1 = 0, idx2 = 1, idx3 = 2;
+
+   for (std::size_t i = 0; i <= jets.size() - n; i++) {
+      auto p1 = jets[i];
+      for (std::size_t j = i + 1; j <= jets.size() - n + 1; j++) {
+         auto p2 = jets[j];
+         for (std::size_t k = j + 1; k <= jets.size() - n + 2; k++) {
+            auto p3 = jets[k];
+            const auto tmp_mass = (p1 + p2 + p3).mass();
+            const auto tmp_distance = std::abs(tmp_mass - top_mass);
+            if (tmp_distance < distance) {
+               distance = tmp_distance;
+               idx1 = i;
+               idx2 = j;
+               idx3 = k;
+            }
+         }
       }
    }
-
-   return {c[0][idx], c[1][idx], c[2][idx]};
+   return {idx1, idx2, idx3};
 }
 
 float benchmark6_trijet_pt(Vec<float> pt, Vec<float> eta, Vec<float> phi, Vec<float> mass, Vec<std::size_t> idx)