diff --git a/src/Storages/StorageMemory.cpp b/src/Storages/StorageMemory.cpp
index 5ef4ab24914..b232ddddd33 100644
--- a/src/Storages/StorageMemory.cpp
+++ b/src/Storages/StorageMemory.cpp
@@ -28,13 +28,13 @@ public:
     MemorySource(
         Names column_names_,
         BlocksList::iterator first_,
-        BlocksList::iterator last_,
+        size_t num_blocks_,
         const StorageMemory & storage,
         const StorageMetadataPtr & metadata_snapshot)
         : SourceWithProgress(metadata_snapshot->getSampleBlockForColumns(column_names_, storage.getVirtuals(), storage.getStorageID()))
         , column_names(std::move(column_names_))
-        , current(first_)
-        , last(last_) /// [first, last]
+        , current_it(first_)
+        , num_blocks(num_blocks_)
     {
     }
 
@@ -49,7 +49,7 @@ protected:
         }
         else
         {
-            const Block & src = *current;
+            const Block & src = *current_it;
             Columns columns;
             columns.reserve(column_names.size());
 
@@ -57,17 +57,23 @@ protected:
             for (const auto & name : column_names)
                 columns.emplace_back(src.getByName(name).column);
 
-            if (current == last)
+            if (current_block_idx == num_blocks)
+            {
                 is_finished = true;
+            }
             else
-                ++current;
+            {
+                ++current_it;
+                ++current_block_idx;
+            }
             return Chunk(std::move(columns), src.rows());
         }
     }
 private:
     Names column_names;
-    BlocksList::iterator current;
-    BlocksList::iterator last;
+    BlocksList::iterator current_it;
+    size_t current_block_idx = 0;
+    const size_t num_blocks;
     bool is_finished = false;
 };
 
@@ -126,27 +132,23 @@ Pipe StorageMemory::read(
 
     Pipes pipes;
 
-    BlocksList::iterator first = data.begin();
+    BlocksList::iterator it = data.begin();
 
     size_t offset = 0;
     for (size_t stream = 0; stream < num_streams; ++stream)
     {
-        auto next = first;
-        while (offset < stream * size / num_streams)
+        size_t next_offset = stream * size / num_streams;
+        size_t num_blocks = next_offset - offset;
+
+        assert(num_blocks > 0);
+
+        pipes.emplace_back(std::make_shared<MemorySource>(column_names, it, num_blocks, *this, metadata_snapshot));
+
+        while (offset < next_offset)
         {
-            ++next;
+            ++it;
             ++offset;
         }
-
-        if (first == next)
-            continue;
-
-        auto last = next;
-        --last;
-
-        pipes.emplace_back(std::make_shared<MemorySource>(column_names, first, last, *this, metadata_snapshot));
-
-        first = next;
     }
 
     return Pipe::unitePipes(std::move(pipes));