Uniform headers for union stream. #2156

dbms/src/Interpreters/InterpreterSelectQuery.cpp

@@ -49,6 +49,7 @@
 #include <Parsers/queryToString.h>
 #include <ext/map.h>
 #include <memory>
+#include <DataStreams/ConvertingBlockInputStream.h>
 
 
 namespace DB
@@ -1293,6 +1294,17 @@ void InterpreterSelectQuery::executeUnion(Pipeline & pipeline)
     /// If there are still several streams, then we combine them into one
     if (pipeline.hasMoreThanOneStream())
     {
+        /// Unify streams in case they have different headers.
+        auto first_header = pipeline.streams.at(0)->getHeader();
+        for (size_t i = 1; i < pipeline.streams.size(); ++i)
+        {
+            auto & stream = pipeline.streams[i];
+            auto header = stream->getHeader();
+            auto mode = ConvertingBlockInputStream::MatchColumnsMode::Name;
+            if (!blocksHaveEqualStructure(first_header, header))
+                stream = std::make_shared<ConvertingBlockInputStream>(context, stream, first_header, mode);
+        }
+
         pipeline.firstStream() = std::make_shared<UnionBlockInputStream<>>(pipeline.streams, pipeline.stream_with_non_joined_data, max_streams);
         pipeline.stream_with_non_joined_data = nullptr;
         pipeline.streams.resize(1);