Optimize single partition spilling (facebookincubator#6271)

Signed-off-by: Yuan Zhou <[email protected]>

velox/exec/Spiller.cpp

@@ -543,20 +543,25 @@ void Spiller::fillSpillRuns(SpillRows* rowsFromNonSpillingPartitions) {
   constexpr int32_t kHashBatchSize = 4096;
   std::vector<uint64_t> hashes(kHashBatchSize);
   std::vector<char*> rows(kHashBatchSize);
+  VELOX_CHECK((type_ != Type::kOrderBy) || bits_.numPartitions() == 1);
+  const bool isSinglePartition =
+    (type_ == Type::kOrderBy || bits_.numPartitions() == 1);
   for (;;) {
     auto numRows = container_->listRows(
         &iterator, rows.size(), RowContainer::kUnlimited, rows.data());
     // Calculate hashes for this batch of spill candidates.
     auto rowSet = folly::Range<char**>(rows.data(), numRows);
-    for (auto i = 0; i < container_->keyTypes().size(); ++i) {
-      container_->hash(i, rowSet, i > 0, hashes.data());
+    if (!isSinglePartition) {
+        for (auto i = 0; i < container_->keyTypes().size(); ++i) {
+          container_->hash(i, rowSet, i > 0, hashes.data());
+        }
     }
 
     // Put each in its run.
     for (auto i = 0; i < numRows; ++i) {
       // TODO: consider to cache the hash bits in row container so we only need
       // to calculate them once.
-      const auto partition = (type_ == Type::kOrderBy)
+      const auto partition = isSinglePartition
           ? 0
           : bits_.partition(hashes[i], state_.maxPartitions());
       VELOX_DCHECK_GE(partition, 0);