返回值
The status can be PGSQL_TRANSACTION_IDLE (currently idle),
    PGSQL_TRANSACTION_ACTIVE (a command is in progress),
    PGSQL_TRANSACTION_INTRANS (idle, in a valid transaction block),
    or PGSQL_TRANSACTION_INERROR (idle, in a failed transaction block).
    PGSQL_TRANSACTION_UNKNOWN is reported if the connection is bad.
    PGSQL_TRANSACTION_ACTIVE is reported only when a query
    has been sent to the server and not yet completed.