[ntuple] conditionally create page lookup data structure

To save space in the cluster descriptor, create the lookup data
structure for large number pages in a cluster+column only if that
cluster+column actually has many (>10) pages.

Reduces the typical size of RPageRange from 56B to 40B.

Author: jblomer

tree/ntuple/inc/ROOT/RNTupleDescriptor.hxx

@@ -445,14 +445,19 @@ public:
       std::size_t ExtendToFitColumnRange(const RColumnRange &columnRange,
                                          const ROOT::Internal::RColumnElementBase &element, std::size_t pageSize);
 
+      std::vector<RPageInfo> fPageInfos;
+
       /// Has the same length than fPageInfos and stores the sum of the number of elements of all the pages
       /// up to and including a given index. Used for binary search in Find().
-      std::vector<ROOT::NTupleSize_t> fCumulativeNElements;
+      /// This vector is only created if fPageInfos has at least kLargeRangeThreshold elements.
+      std::unique_ptr<std::vector<ROOT::NTupleSize_t>> fCumulativeNElements;
 
       ROOT::DescriptorId_t fPhysicalColumnId = ROOT::kInvalidDescriptorId;
-      std::vector<RPageInfo> fPageInfos;
 
    public:
+      /// Create the fCumulativeNElements only when its needed, i.e. when there are many pages to search through.
+      static constexpr std::size_t kLargeRangeThreshold = 10;
+
       RPageRange() = default;
       RPageRange(const RPageRange &other) = delete;
       RPageRange &operator=(const RPageRange &other) = delete;
@@ -464,7 +469,9 @@ public:
          RPageRange clone;
          clone.fPhysicalColumnId = fPhysicalColumnId;
          clone.fPageInfos = fPageInfos;
-         clone.fCumulativeNElements = fCumulativeNElements;
+         if (fCumulativeNElements) {
+            clone.fCumulativeNElements = std::make_unique<std::vector<ROOT::NTupleSize_t>>(*fCumulativeNElements);
+         }
          return clone;
       }
 

tree/ntuple/src/RNTupleDescriptor.cxx

@@ -208,7 +208,21 @@ ROOT::RColumnDescriptor ROOT::RColumnDescriptor::Clone() const
 ROOT::RClusterDescriptor::RPageInfoExtended
 ROOT::RClusterDescriptor::RPageRange::Find(ROOT::NTupleSize_t idxInCluster) const
 {
-   const auto N = fCumulativeNElements.size();
+   if (!fCumulativeNElements) {
+      // Small range, just iterate through fPageInfos
+      NTupleSize_t pageNumber = 0;
+      NTupleSize_t firstInPage = 0;
+      for (const auto &pi : fPageInfos) {
+         if (firstInPage + pi.GetNElements() > idxInCluster) {
+            return RPageInfoExtended{pi, firstInPage, pageNumber};
+         }
+         pageNumber++;
+         firstInPage += pi.GetNElements();
+      }
+      R__ASSERT(false);
+   }
+
+   const auto N = fCumulativeNElements->size();
    R__ASSERT(N > 0);
    R__ASSERT(N == fPageInfos.size());
 
@@ -217,20 +231,20 @@ ROOT::RClusterDescriptor::RPageRange::Find(ROOT::NTupleSize_t idxInCluster) cons
    std::size_t midpoint = N;
    while (left <= right) {
       midpoint = (left + right) / 2;
-      if (fCumulativeNElements[midpoint] <= idxInCluster) {
+      if ((*fCumulativeNElements)[midpoint] <= idxInCluster) {
          left = midpoint + 1;
          continue;
       }
 
-      if ((midpoint == 0) || (fCumulativeNElements[midpoint - 1] <= idxInCluster))
+      if ((midpoint == 0) || ((*fCumulativeNElements)[midpoint - 1] <= idxInCluster))
          break;
 
       right = midpoint - 1;
    }
    R__ASSERT(midpoint < N);
 
    auto pageInfo = fPageInfos[midpoint];
-   decltype(idxInCluster) firstInPage = (midpoint == 0) ? 0 : fCumulativeNElements[midpoint - 1];
+   decltype(idxInCluster) firstInPage = (midpoint == 0) ? 0 : (*fCumulativeNElements)[midpoint - 1];
    R__ASSERT(firstInPage <= idxInCluster);
    R__ASSERT((firstInPage + pageInfo.GetNElements()) > idxInCluster);
    return RPageInfoExtended{pageInfo, firstInPage, midpoint};
@@ -975,12 +989,16 @@ ROOT::RResult<ROOT::RClusterDescriptor> ROOT::Internal::RClusterDescriptorBuilde
       if (fCluster.fColumnRanges.count(pr.first) == 0) {
          return R__FAIL("missing column range");
       }
-      pr.second.fCumulativeNElements.clear();
-      pr.second.fCumulativeNElements.reserve(pr.second.fPageInfos.size());
-      ROOT::NTupleSize_t sum = 0;
-      for (const auto &pi : pr.second.fPageInfos) {
-         sum += pi.GetNElements();
-         pr.second.fCumulativeNElements.emplace_back(sum);
+      pr.second.fCumulativeNElements.reset();
+      const auto nPages = pr.second.fPageInfos.size();
+      if (nPages > RClusterDescriptor::RPageRange::kLargeRangeThreshold) {
+         pr.second.fCumulativeNElements = std::make_unique<std::vector<NTupleSize_t>>();
+         pr.second.fCumulativeNElements->reserve(nPages);
+         ROOT::NTupleSize_t sum = 0;
+         for (const auto &pi : pr.second.fPageInfos) {
+            sum += pi.GetNElements();
+            pr.second.fCumulativeNElements->emplace_back(sum);
+         }
       }
    }
    RClusterDescriptor result;