Обсуждение: Using HeapTuple.t_len to verify size of tuple

Vignesh Raghunathan <vignesh.pgsql@gmail.com> writes:
> Can the t_len field in HeapTuple structure be used to verify the length of
> the tuple?

> That is, if I calculate the length from the contents of the tuple using
> data from pg_attribute for fixed length fields and the data from the header
> for varlena fields, should it always match the value stored in t_len?

If t_len were *less* than that, it would be a bug.  But I think it's
fairly common for t_len to be rounded up to the next maxalign boundary.
        regards, tom lane

Vignesh Raghunathan <vignesh.pgsql@gmail.com> writes:
> On Tue, Aug 25, 2015 at 2:56 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> If t_len were *less* than that, it would be a bug.  But I think it's
>> fairly common for t_len to be rounded up to the next maxalign boundary.

> I have modified heap_deform_tuple code to check whether the 'off' variable
> matches the tuple's t_len field for a project. In the code, off is not
> updated in case of null fields. However, when I run it for pg_class table,
> the code throws an error saying that the value of off does not match t_len.
> It turns out that for all tuples even if the attribute attacl is null,
> t_len field is set to be the sizeof(FormData_pg_class).

You mean "even if relacl is not null"?  Sounds improbable: AFAIR, pg_class
tuples are built with heap_form_tuple, same as anything else.
        regards, tom lane