Você pode escrever algo assim:
SELECT (t2.function_row).id,
(t2.function_row).cost
FROM (SELECT _tmp_myfunction(t.id) as function_row
FROM transactions t ) t2;
Ele fornecerá os campos, em vez de linhas compostas.
SELECT (t2.function_row).id,
(t2.function_row).cost
FROM (SELECT _tmp_myfunction(t.id) as function_row
FROM transactions t ) t2;