CLICKHOUSE-4127: Fix assertion in debug build.

Do reading faster if the structure isn't altered.

dbms/src/Storages/StorageBuffer.cpp

+#include <boost/range/algorithm_ext/erase.hpp>
 #include <Interpreters/InterpreterSelectQuery.h>
 #include <Interpreters/InterpreterInsertQuery.h>
 #include <Interpreters/InterpreterAlterQuery.h>
@@ -148,56 +149,65 @@ BlockInputStreams StorageBuffer::read(
         if (destination.get() == this)
             throw Exception("Destination table is myself. Read will cause infinite loop.", ErrorCodes::INFINITE_LOOP);
 
-        /// Collect columns from the destination tables which can be requested.
-        /// Find out if there is a struct mismatch and we need to convert read blocks from the destination tables.
         auto destination_lock = destination->lockStructure(false);
 
-        Names columns_intersection;
-        bool struct_mismatch = false;
-        for (const String & column_name : column_names)
+        const bool dst_has_same_structure = std::all_of(column_names.begin(), column_names.end(), [this, destination](const String& column_name)
         {
-            if (destination->hasColumn(column_name))
+            return destination->hasColumn(column_name) &&
+                   destination->getColumn(column_name).type->equals(*getColumn(column_name).type);
+        });
+
+        if (dst_has_same_structure)
+        {
+            /// The destination table has the same structure of the requested columns and we can simply read blocks from there.
+            streams_from_dst = destination->read(column_names, query_info, context, processed_stage, max_block_size, num_streams);
+        }
+        else
+        {
+            /// There is a struct mismatch and we need to convert read blocks from the destination table.
+            const Block header = getSampleBlock();
+            Names columns_intersection = column_names;
+            Block header_after_adding_defaults = header;
+            for (const String & column_name : column_names)
             {
-                columns_intersection.emplace_back(column_name);
-                if (!destination->getColumn(column_name).type->equals(*getColumn(column_name).type))
+                if (!destination->hasColumn(column_name))
+                {
+                    LOG_WARNING(log, "Destination table " << backQuoteIfNeed(destination_database) << "." << backQuoteIfNeed(destination_table)
+                        << " doesn't have column " << backQuoteIfNeed(column_name) << ". The default values are used.");
+                    boost::range::remove_erase(columns_intersection, column_name);
+                    continue;
+                }
+                const auto & dst_col = destination->getColumn(column_name);
+                const auto & col = getColumn(column_name);
+                if (!dst_col.type->equals(*col.type))
                 {
                     LOG_WARNING(log, "Destination table " << backQuoteIfNeed(destination_database) << "." << backQuoteIfNeed(destination_table)
                         << " has different type of column " << backQuoteIfNeed(column_name) << " ("
-                        << destination->getColumn(column_name).type->getName() << " != " << getColumn(column_name).type->getName()
-                        << "). Data from destination table is converted.");
-                    struct_mismatch = true;
+                        << dst_col.type->getName() << " != " << col.type->getName() << "). Data from destination table are converted.");
+                    header_after_adding_defaults.getByName(column_name) = ColumnWithTypeAndName(dst_col.type, column_name);
                 }
             }
-            else
+
+            if (columns_intersection.empty())
             {
                 LOG_WARNING(log, "Destination table " << backQuoteIfNeed(destination_database) << "." << backQuoteIfNeed(destination_table)
-                    << " doesn't have column " << backQuoteIfNeed(column_name) << ". The default values are used.");
-                struct_mismatch = true;
+                    << " has no common columns with block in buffer. Block of data is skipped.");
             }
-        }
-
-        if (columns_intersection.empty())
-            LOG_WARNING(log, "Destination table " << backQuoteIfNeed(destination_database) << "." << backQuoteIfNeed(destination_table)
-                << " has no common columns with block in buffer. Block of data is skipped.");
-        else
-        {
-            streams_from_dst = destination->read(columns_intersection, query_info, context, processed_stage, max_block_size, num_streams);
-            for (auto & stream : streams_from_dst)
-                stream->addTableLock(destination_lock);
-        }
-
-        if (struct_mismatch && !streams_from_dst.empty())
-        {
-            /// Add streams to convert read blocks from the destination table.
-            auto header = getSampleBlock();
-            for (auto & stream_from_dst : streams_from_dst)
+            else
             {
-                stream_from_dst = std::make_shared<AddingDefaultBlockInputStream>(
-                            stream_from_dst, header, getColumns().defaults, context);
-                stream_from_dst = std::make_shared<ConvertingBlockInputStream>(
-                            context, stream_from_dst, header, ConvertingBlockInputStream::MatchColumnsMode::Name);
+                streams_from_dst = destination->read(columns_intersection, query_info, context, processed_stage, max_block_size, num_streams);
+                for (auto & stream : streams_from_dst)
+                {
+                    stream = std::make_shared<AddingDefaultBlockInputStream>(
+                                stream, header_after_adding_defaults, getColumns().defaults, context);
+                    stream = std::make_shared<ConvertingBlockInputStream>(
+                                context, stream, header, ConvertingBlockInputStream::MatchColumnsMode::Name);
+                }
             }
         }
+
+        for (auto & stream : streams_from_dst)
+            stream->addTableLock(destination_lock);
     }
 
     BlockInputStreams streams_from_buffers;