PostgreSQL - como obtengo la primera factura de un cliente

select *

from wallet.transaction

where transaction_type_id = 1

and transaction_status_id = 6

and customer_id = ''

order by completed asc limit 1

WITH t_customers AS (

   SELECT DISTINCT customer_id

       FROM wallet.transaction

       WHERE customer_id = ''

)

SELECT

    c.customer_id,

    (SELECT factura

     FROM wallet.transaction wt

        WHERE wt.customer_id = c.customer_id

            AND transaction_type_id = 1

            AND transaction_status_id = 6

     ORDER by factura LIMIT 1) primer_factura

FROM t_customers c