Add estimateCompact size API

velox/vector/BaseVector.cpp

@@ -828,6 +828,14 @@ uint64_t BaseVector::estimateFlatSize() const {
   return length_ * avgRowSize;
 }
 
+double BaseVector::estimateRowSize() const {
+    return estimateFlatSize() / length_;
+}
+
+uint64_t BaseVector::estimateCompactSize() const {
+  return estimateFlatSize();
+}
+
 namespace {
 bool isReusableEncoding(VectorEncoding::Simple encoding) {
   return encoding == VectorEncoding::Simple::FLAT ||

velox/vector/BaseVector.h

@@ -727,6 +727,15 @@ class BaseVector {
   /// hasn't been loaded yet.
   virtual uint64_t estimateFlatSize() const;
 
+  /// Returns an estimate size of each row in this vector.
+  virtual double estimateRowSize() const;
+
+  /// Returns an estimate size of the vector as if it was compacted, ignoring any over-allocations. For example:
+  /// (1) For dictionary vector, it only counts each dictionary entry once, rather than each time a value is referenced.
+  /// (2) For flat vector, returns its retained size.
+  /// (3) For lazy vector that hasn't been loaded, returns zero.
+  virtual uint64_t estimateCompactSize() const;
+
   /// To safely reuse a vector one needs to (1) ensure that the vector as well
   /// as all its buffers and child vectors are singly-referenced and mutable
   /// (for buffers); (2) clear append-only string buffers and child vectors

velox/vector/ConstantVector.h

@@ -205,6 +205,14 @@ class ConstantVector final : public SimpleVector<T> {
     return sizeof(T);
   }
 
+  double estimateRowSize() const override {
+    return valueVector_ ? valueVector_->estimateRowSize() : retainedSize();
+  }
+
+  uint64_t estimateCompactSize() const override {
+    return 1 * estimateRowSize();
+  }
+
   BaseVector* loadedVector() override {
     if (!valueVector_) {
       return this;

velox/vector/DictionaryVector.h

@@ -148,6 +148,25 @@ class DictionaryVector : public SimpleVector<T> {
         indices_->capacity();
   }
 
+  double estimateRowSize() const override {
+    return 1.0 * dictionaryValues_->estimateRowSize();
+  }
+
+  uint64_t estimateCompactSize() const override {
+    if (indices() == nullptr) {
+      return 0;
+    }
+
+    std::unordered_set<int32_t> distinctCount;
+    const int32_t* rawIndices = indices_->as<int32_t>();
+    for (int i = 0; i < BaseVector::length_; i++) {
+      if (!BaseVector::isNullAt(i)) {
+        distinctCount.insert(rawIndices[i]);
+      }
+    }
+    return distinctCount.size() * estimateRowSize();
+  }
+
   bool isScalar() const override {
     return dictionaryValues_->isScalar();
   }

velox/vector/FlatVector.h

@@ -386,6 +386,17 @@ class FlatVector final : public SimpleVector<T> {
     return size;
   }
 
+  double estimateRowSize() const override {
+    if (BaseVector::length_ == 0) {
+      return 0;
+    }
+    return 1.0 * retainedSize() / BaseVector::length_;
+  }
+
+  uint64_t estimateCompactSize() const override {
+    return retainedSize();
+  }
+
   /**
    * Used for vectors of type VARCHAR and VARBINARY to hold data referenced by
    * StringView's. It is safe to share these among multiple vectors. These

velox/vector/SequenceVector.h

@@ -153,6 +153,14 @@ class SequenceVector : public SimpleVector<T> {
     return sequenceValues_->retainedSize() + sequenceLengths_->capacity();
   }
 
+  double estimateRowSize() const override {
+    return sequenceValues_->estimateRowSize();
+  }
+
+  uint64_t estimateCompactSize() const override {
+    return sequenceLengths_->size() * estimateRowSize();
+  }
+
   bool isScalar() const override {
     return sequenceValues_->isScalar();
   }