Re: patch submission: truncate trailing nulls from heap rows to reduce the size of the null bitmap [Review]

On Monday, June 24, 2013 8:20 PM Tom Lane wrote:
> Amit Kapila <amit(dot)kapila(at)huawei(dot)com> writes:
> > I will summarize the results, and if most of us feel that they are
> not good
> > enough, then we can return this patch.
>
> Aside from the question of whether there's really any generally-useful
> performance improvement from this patch, it strikes me that this change
> forecloses other games that we might want to play with interpretation
> of
> the value of a tuple's natts.
>
> In particular, when I was visiting Salesforce last week, the point came
> up that they'd really like ALTER TABLE ADD COLUMN to be "free" even
> when
> the column had a non-null DEFAULT. It's not too difficult to imagine
> how we might support that, at least when the default expression is a
> constant: decree that if the requested attribute number is beyond
> natts,
> look aside at the tuple descriptor to see if the column is marked as
> having a magic default value, and if so, substitute that rather than
> just returning NULL. (This has to be a "magic" value separate from
> the current default, else a subsequent DROP or SET DEFAULT would do
> the wrong thing.)
>
> However, this idea conflicts with an optimization that supposes it can
> reduce natts to suppress null columns: if the column was actually
> stored
> as null, you'd lose that information, and would incorrectly return the
> magic default on subsequent reads.
>
> I think it might be possible to make both ideas play together, by
> not reducing natts further than the last column with a magic default.
> However, that means extra complexity in heap_form_tuple, which would
> invalidate the performance measurements done in support of this patch.

It can have slight impact on normal scenario's, but I am not sure how much
because
the change will be very less(may be one extra if check and one assignment)

For this Patch's scenario, I think the major benefit for Performance is in
heap_fill_tuple() where the
For loop is reduced. However added some logic in heap_form_tuple can
reduce the performance improvement,
but there can still be space saving benefit.

With Regards,
Amit Kapila.