#3865 fix performance regression introduced by MergeOperator.ShouldMerge (#4266)

Summary:
This PR addresses issue #3865 and implements the following approach to fix it:
 - adds `MergeContext::GetOperandsDirectionForward` and `MergeContext::GetOperandsDirectionBackward` to query merge operands in a specific order
 - `MergeContext::GetOperands` becomes a shortcut for `MergeContext::GetOperandsDirectionForward`
 - pass `MergeContext::GetOperandsDirectionBackward` to `MergeOperator::ShouldMerge` and document the order
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4266

Differential Revision: D9360750

Pulled By: sagar0

fbshipit-source-id: 20cb73ff017760b062ecdcf4382560767086e092

HISTORY.md

@@ -1,6 +1,7 @@
 # Rocksdb Change Log
 ## Unreleased
 ### Public API Change
+* The merge operands are passed to `MergeOperator::ShouldMerge` in the reversed order relative to how they were merged (passed to FullMerge or FullMergeV2) for performance reasons 
 ### New Features
 * Changes the format of index blocks by delta encoding the index values, which are the block handles. This saves the encoding of BlockHandle::offset of the non-head index entries in each restart interval. The feature is backward compatible but not forward compatible. It is disabled by default unless format_version 4 or above is used.
 * Add a new tool: trace_analyzer. Trace_analyzer analyzes the trace file generated by using trace_replay API. It can convert the binary format trace file to a human readable txt file, output the statistics of the analyzed query types such as access statistics and size statistics, combining the dumped whole key space file to analyze, support query correlation analyzing, and etc. Current supported query types are: Get, Put, Delete, SingleDelete, DeleteRange, Merge, Iterator (Seek, SeekForPrev only).

db/memtable.cc

@@ -697,7 +697,7 @@ static bool SaveValue(void* arg, const char* entry) {
         *(s->merge_in_progress) = true;
         merge_context->PushOperand(
             v, s->inplace_update_support == false /* operand_pinned */);
-        if (merge_operator->ShouldMerge(merge_context->GetOperands())) {
+        if (merge_operator->ShouldMerge(merge_context->GetOperandsDirectionBackward())) {
           *(s->status) = MergeHelper::TimedFullMerge(
               merge_operator, s->key->user_key(), nullptr,
               merge_context->GetOperands(), s->value, s->logger, s->statistics,

db/merge_context.h

@@ -74,8 +74,13 @@ class MergeContext {
     return (*operand_list_)[index];
   }
 
-  // Return all the operands.
+  // Same as GetOperandsDirectionForward
   const std::vector<Slice>& GetOperands() {
+    return GetOperandsDirectionForward();
+  }
+
+  // Return all the operands in the order as they were merged (passed to FullMerge or FullMergeV2)
+  const std::vector<Slice>& GetOperandsDirectionForward() {
     if (!operand_list_) {
       return empty_operand_list;
     }
@@ -84,6 +89,16 @@ class MergeContext {
     return *operand_list_;
   }
 
+  // Return all the operands in the reversed order relative to how they were merged (passed to FullMerge or FullMergeV2)
+  const std::vector<Slice>& GetOperandsDirectionBackward() {
+    if (!operand_list_) {
+      return empty_operand_list;
+    }
+
+    SetDirectionBackward();
+    return *operand_list_;
+  }
+
  private:
   void Initialize() {
     if (!operand_list_) {

include/rocksdb/merge_operator.h

@@ -195,6 +195,11 @@ class MergeOperator {
   // during a point lookup, thereby helping in limiting the number of levels to
   // read from.
   // Doesn't help with iterators.
+  //
+  // Note: the merge operands are passed to this function in the reversed order
+  // relative to how they were merged (passed to FullMerge or FullMergeV2)
+  // for performance reasons, see also:
+  // https://github.com/facebook/rocksdb/issues/3865
   virtual bool ShouldMerge(const std::vector<Slice>& /*operands*/) const {
     return false;
   }