Merge pull request #15871 from ClickHouse/backport/20.8/15662

Backport #15662 to 20.8: Fix read from Buffer table with empty nested table with different types.

src/Storages/StorageBuffer.cpp

@@ -221,17 +221,20 @@ Pipe StorageBuffer::read(
                     columns_intersection, destination_metadata_snapshot, query_info,
                     context, processed_stage, max_block_size, num_streams);
 
-                pipe_from_dst.addSimpleTransform([&](const Block & stream_header)
+                if (!pipe_from_dst.empty())
                 {
-                    return std::make_shared<AddingMissedTransform>(stream_header, header_after_adding_defaults,
-                        metadata_snapshot->getColumns(), context);
-                });
-
-                pipe_from_dst.addSimpleTransform([&](const Block & stream_header)
-                {
-                    return std::make_shared<ConvertingTransform>(
-                        stream_header, header, ConvertingTransform::MatchColumnsMode::Name);
-                });
+                    pipe_from_dst.addSimpleTransform([&](const Block & stream_header)
+                    {
+                        return std::make_shared<AddingMissedTransform>(stream_header, header_after_adding_defaults,
+                            metadata_snapshot->getColumns(), context);
+                    });
+
+                    pipe_from_dst.addSimpleTransform([&](const Block & stream_header)
+                    {
+                        return std::make_shared<ConvertingTransform>(
+                            stream_header, header, ConvertingTransform::MatchColumnsMode::Name);
+                    });
+                }
             }
         }
 

tests/queries/0_stateless/01514_empty_buffer_different_types.reference

@@ -0,0 +1 @@
+a

tests/queries/0_stateless/01514_empty_buffer_different_types.sql

@@ -0,0 +1,12 @@
+set send_logs_level = 'error';
+
+DROP TABLE IF EXISTS merge_tree_table1;
+CREATE TABLE merge_tree_table1 (`s` LowCardinality(String), x UInt32) ENGINE = MergeTree ORDER BY x settings index_granularity = 1;
+CREATE TABLE buffer_table1 ( `s` String , x UInt32) ENGINE = Buffer(currentDatabase(), 'merge_tree_table1', 16, 10, 60, 10, 1000, 1048576, 2097152);
+SELECT s FROM buffer_table1;
+
+insert into merge_tree_table1 values ('a', 1);
+select s from buffer_table1 where x = 1;
+select s from buffer_table1 where x = 2;
+
+DROP TABLE IF EXISTS merge_tree_table1;