Merge pull request ClickHouse#80328 from bigo-sg/opt_concat_blocks

Trivial opt on `concatenateBlocks`. Maybe it's good for parallel hash join.

Author: Algunenano

src/Core/Block.cpp

@@ -943,6 +943,9 @@ Block concatenateBlocks(const std::vector<Block> & blocks)
     if (blocks.empty())
         return {};
 
+    if (blocks.size() == 1)
+        return blocks[0];
+
     size_t num_rows = 0;
     for (const auto & block : blocks)
         num_rows += block.rows();

src/Interpreters/GraceHashJoin.cpp

@@ -64,15 +64,11 @@ namespace
                 if (!block)
                 {
                     eof = true;
-                    if (blocks.size() == 1)
-                        return blocks.front();
                     return concatenateBlocks(blocks);
                 }
                 blocks.push_back(std::move(block));
             } while (rows_read < result_block_size);
 
-            if (blocks.size() == 1)
-                return blocks.front();
             return concatenateBlocks(blocks);
         }
 
@@ -743,10 +739,7 @@ void GraceHashJoin::addBlockToJoinImpl(Block block)
                 current_blocks.emplace_back(std::move(blocks[bucket_index]));
             }
 
-            if (current_blocks.size() == 1)
-                current_block = std::move(current_blocks.front());
-            else
-                current_block = concatenateBlocks(current_blocks);
+            current_block = concatenateBlocks(current_blocks);
         }
 
         hash_join = makeInMemoryJoin(fmt::format("grace{}", bucket_index), prev_keys_num);

src/Interpreters/SortedBlocksWriter.cpp

@@ -389,9 +389,6 @@ Block SortedBlocksBuffer::mergeBlocks(Blocks && blocks) const
         blocks.swap(tmp_blocks);
     }
 
-    if (blocks.size() == 1)
-        return blocks[0];
-
     return concatenateBlocks(blocks);
 }
 

src/TableFunctions/TableFunctionExplain.cpp

@@ -167,9 +167,6 @@ Block executeMonoBlock(QueryPipeline & pipeline)
             break;
     }
 
-    if (blocks.size() == 1)
-        return blocks[0];
-
     return concatenateBlocks(blocks);
 }
 

src/TableFunctions/TableFunctionFormat.cpp

@@ -125,9 +125,6 @@ Block TableFunctionFormat::parseData(const ColumnsDescription & columns, const S
     while (reader->pull(block))
         blocks.push_back(std::move(block));
 
-    if (blocks.size() == 1)
-        return blocks[0];
-
     /// In case when data contains more then 1 block we combine
     /// them all to one big block (this is considered a rare case).
     return concatenateBlocks(blocks);