Re: Using HeapTuple.t_len to verify size of tuple

On Tue, Aug 25, 2015 at 2:56 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Vignesh Raghunathan <vignesh(dot)pgsql(at)gmail(dot)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

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). Is this normal
behavior?