Method evaluateSharedSubexpr

bolt/expression/Expr.cpp:880–977

template <typename TEval>
void Expr::evaluateSharedSubexpr(
    const SelectivityVector& rows,
    EvalCtx& context,
    VectorPtr& result,
    TEval eval) {
  // Captures the inputs referenced by distinctFields_.
  std::vector<const BaseVector*> expressionInputFields;
  for (auto* field : distinctFields_) {
    expressionInputFields.push_back(
        context.getField(field->index(context)).get());
  }

  // Find the cached results for the same inputs, or create an entry if one
  // doesn't exist.
  auto sharedSubexprResultsIter =
      sharedSubexprResults_.find(expressionInputFields);
  if (sharedSubexprResultsIter == sharedSubexprResults_.end()) {
    auto maxSharedSubexprResultsCached = context.execCtx()
                                             ->queryCtx()
                                             ->queryConfig()
                                             .maxSharedSubexprResultsCached();
    if (sharedSubexprResults_.size() < maxSharedSubexprResultsCached) {
      // If we have room left in the cache, add it.
      sharedSubexprResultsIter =
          sharedSubexprResults_
              .insert(
                  std::pair(std::move(expressionInputFields), SharedResults()))
              .first;
    } else {
      // Otherwise, simply evaluate it and return without caching the results.
      eval(rows, context, result);

      return;
    }
  }

  auto& [sharedSubexprRows, sharedSubexprValues] =
      sharedSubexprResultsIter->second;

  if (sharedSubexprValues == nullptr) {
    eval(rows, context, result);

    if (!sharedSubexprRows) {
      sharedSubexprRows = context.execCtx()->getSelectivityVector(rows.end());
    }

    *sharedSubexprRows = rows;
    if (context.errors()) {
      // Clear the rows which failed to compute.
      context.deselectErrors(*sharedSubexprRows);
      if (!sharedSubexprRows->hasSelections()) {
        // Do not store a reference to 'result' if we cannot use any rows from
        // it.
        return;
      }
    }

    sharedSubexprValues = result;