Re: Fast ALTER TABLE ... ADD COLUMN ... DEFAULT xxx?

>
> Dmitry Koterov <dmitry(at)koterov(dot)ru> writes:
> > No, I meant that in case of the row (1, NULL, NULL, 2, 3, NULL):
> > - the corresponding NULL bitmap is (100110...)
> > - the corresponding tuple is (1, 2, 3)
> > - t_natts=3 (if I am not wrong here)
>
> You are wrong --- t_natts would be six here. In general the length of
> the null bitmap in a tuple (if it has one at all) is always exactly
> equal to its t_natts value.
>

And so, the real number of values in the tuple - (1, 2, 3) above - is equal
to the number of 1-bits in NULL bitmap. And the size of NULL bitmap is held
in t_natts. I meant that when I said "thanks to NULL bitmap, adding a new
nullable column is cheap". :-) And, of course, thanks to t_natts
(HeapTupleHeaderGetNatts macro) - too.