added postgres backend

backends/postgres/include/postgres_connection.hpp

@@ -0,0 +1,51 @@
+#ifndef QUERY_POSTGRES_CONNECTION_HPP
+#define QUERY_POSTGRES_CONNECTION_HPP
+
+#ifdef _MSC_VER
+#ifdef matador_postgres_EXPORTS
+#define MATADOR_POSTGRES_API __declspec(dllexport)
+#else
+#define MATADOR_POSTGRES_API __declspec(dllimport)
+#endif
+#pragma warning(disable: 4355)
+#else
+#define MATADOR_POSTGRES_API
+#endif
+
+#include "matador/sql/connection_impl.hpp"
+
+#include <libpq-fe.h>
+
+namespace matador::backends::postgres {
+
+class postgres_connection : public matador::sql::connection_impl
+{
+public:
+  explicit postgres_connection(const sql::connection_info &info);
+  void open() override;
+  void close() override;
+  bool is_open() override;
+
+  std::unique_ptr<sql::query_result_impl> fetch(const std::string &stmt) override;
+  void prepare(const std::string &stmt) override;
+
+  size_t execute(const std::string &stmt) override;
+
+  sql::record describe(const std::string& table) override;
+
+  bool exists(const std::string &table_name) override;
+
+private:
+  PGconn *conn_{nullptr};
+};
+
+}
+
+extern "C"
+{
+MATADOR_POSTGRES_API matador::sql::connection_impl* create_database(const matador::sql::connection_info &info);
+
+MATADOR_POSTGRES_API void destroy_database(matador::sql::connection_impl *db);
+}
+
+#endif //QUERY_POSTGRES_CONNECTION_HPP