added mysql backend

backends/postgres/src/postgres_connection.cpp

@@ -161,9 +161,9 @@ sql::record postgres_connection::describe(const std::string &table)
   return std::move(prototype);
 }
 
-bool postgres_connection::exists(const std::string &table_name)
+bool postgres_connection::exists(const std::string &schema_name, const std::string &table_name)
 {
-  std::string stmt("SELECT 1 FROM information_schema.tables WHERE table_schema = 'public' AND table_name = '" + table_name + "'");
+  std::string stmt("SELECT 1 FROM information_schema.tables WHERE table_schema = '" + schema_name + "' AND table_name = '" + table_name + "'");
 
   PGresult *res = PQexec(conn_, stmt.c_str());