Description

The [MinKey, MaxKey] fast path in ChunkManager::getShardIdsForRange() is missing its return;, so it populates the output and then falls through into forEachOverlappingChunk(), which re-scans every chunk in the collection. The optimization is fully defeated. The comment ("return all shard ids right away") confirms
the return was intended.

chunk_manager.cpp#L844-L850

      if (!_clusterTime && ChunkMap::allElementsAreOfType(BSONType::minKey, min) &&
          ChunkMap::allElementsAreOfType(BSONType::maxKey, max)) {
          _rt->optRt->getAllShardIds(shardIds);
          if (chunkRanges) {
              getAllChunkRanges(chunkRanges);
          }
      }   // <-- missing `return;` (line 850)
  

Impact

Performance only; results stay correct. Turns ~O(#shards) into an O(#chunks) full scan on a common query shape (whole-collection targeting).

Fix

Add return; at line 850.