Add support for prepared statement SELECT without parameters (#179)

Closes GH-178

Author: kou

src/afs.cc

@@ -1309,6 +1309,19 @@ class PreparedStatement {
 	using WriteFunc = std::add_pointer<arrow::Status(void*)>::type;
 	arrow::Status select(WriteFunc write, void* writeData)
 	{
+		if (parameters_.empty())
+		{
+			auto result = SPI_execute(query_.c_str(), false, 0);
+			if (result <= 0)
+			{
+				return arrow::Status::Invalid("failed to run a query: ",
+				                              SPI_result_code_string(result),
+				                              ": ",
+				                              query_);
+			}
+			return write(writeData);
+		}
+
 		for (const auto& recordBatch : parameters_)
 		{
 			SPIExecuteOptions options = {};

test/test-flight-sql.rb

@@ -114,6 +114,22 @@ def test_select_prepare
     end
   end
 
+  def test_select_prepare_without_parameters
+    unless flight_sql_client.respond_to?(:prepare)
+      omit("red-arrow-flight-sql 14.0.0 or later is required")
+    end
+
+    flight_sql_client.prepare("SELECT 29 AS value", @options) do |statement|
+      info = statement.execute(@options)
+      assert_equal(Arrow::Schema.new(value: :int32),
+                   info.get_schema)
+      endpoint = info.endpoints.first
+      reader = flight_sql_client.do_get(endpoint.ticket, @options)
+      assert_equal(Arrow::Table.new(value: Arrow::Int32Array.new([29])),
+                   reader.read_all)
+    end
+  end
+
   def test_select_from
     run_sql("CREATE TABLE data (value integer)")
     run_sql("INSERT INTO data VALUES (1), (-2), (3)")