Improve format JSONColumns when result is empty

src/Processors/Formats/Impl/JSONColumnsBlockOutputFormat.cpp

@@ -8,7 +8,7 @@ namespace DB
 {
 
 JSONColumnsBlockOutputFormat::JSONColumnsBlockOutputFormat(WriteBuffer & out_, const Block & header_, const FormatSettings & format_settings_, bool validate_utf8, size_t indent_)
-    : JSONColumnsBlockOutputFormatBase(out_, header_, format_settings_, validate_utf8), indent(indent_)
+    : JSONColumnsBlockOutputFormatBase(out_, header_, format_settings_, validate_utf8), indent(indent_), header(header_)
 {
     names = JSONUtils::makeNamesValidJSONStrings(header_.getNames(), format_settings, validate_utf8);
 }
@@ -25,6 +25,18 @@ void JSONColumnsBlockOutputFormat::writeColumnStart(size_t column_index)
 
 void JSONColumnsBlockOutputFormat::writeChunkEnd()
 {
+    /// Write empty chunk
+    if (!written_rows)
+    {
+        const auto & columns = header.getColumns();
+        for (size_t i = 0; i != columns.size(); ++i)
+        {
+            writeColumnStart(i);
+            writeColumn(*columns[i], *serializations[i]);
+            writeColumnEnd(i == columns.size() - 1);
+        }
+    }
+
     JSONUtils::writeObjectEnd(*ostr, indent);
     writeChar('\n', *ostr);
 }

src/Processors/Formats/Impl/JSONColumnsBlockOutputFormat.h

@@ -27,6 +27,8 @@ protected:
 
     Names names;
     size_t indent;
+
+    Block header;
 };
 
 }

src/Processors/Formats/Impl/JSONColumnsBlockOutputFormatBase.cpp

@@ -42,6 +42,7 @@ void JSONColumnsBlockOutputFormatBase::writeChunk(Chunk & chunk)
         writeColumn(*columns[i], *serializations[i]);
         writeColumnEnd(i == columns.size() - 1);
     }
+    written_rows += chunk.getNumRows();
     writeChunkEnd();
 }
 

src/Processors/Formats/Impl/JSONColumnsBlockOutputFormatBase.h

@@ -36,6 +36,8 @@ protected:
     const Serializations serializations;
 
     Chunk mono_chunk;
+
+    size_t written_rows = 0;
 };
 
 }

tests/queries/0_stateless/02554_format_json_columns_for_empty.reference

@@ -0,0 +1,4 @@
+{
+	"n": [],
+	"s": []
+}

tests/queries/0_stateless/02554_format_json_columns_for_empty.sql

@@ -0,0 +1,5 @@
+DROP TABLE IF EXISTS json_columns;
+
+CREATE TABLE json_columns (n UInt32, s String) ENGINE = MergeTree order by n;
+
+SELECT * FROM json_columns FORMAT JSONColumns;