Re: Fix tuple deformation with virtual generated NOT NULL columns

> While testing "Optimize tuple deformation”, I found a bug:
> ```
> evantest=# create table t (a int not null,
> evantest(# g int generated always as (a+1) virtual not null,
> evantest(# b int not null);
> CREATE TABLE
> evantest=# insert into t (a, b) values (10, 20);
> INSERT 0 1
> evantest=# select a, g, b from t;
>  a  | g  | b
> ----+----+---
>  10 | 11 | 0
> (1 row)
> ```

Nice catch! I can reproduce this bug on master. Some comments about the fix:

I find that a virtual generated column is stored as a null in heap tuple, so I think
we should stop setting 'attcacheoff' when we see a virtual generated column in
TupleDescFinalize(), or we will set wrong 'attcacheoff' value. But it seems that
we don't use these wrong value because we can only use 'attcacheoff' up until
the first NULL.

--
Regards,
ChangAo Chen