Re: TOAST condition for column size

Dilip Kumar <dilipbalaut(at)gmail(dot)com> writes:
> On Tue, 19 Jan 2021 at 6:28 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> Won't it be safe because we don't align individual attrs of type
>> varchar where length is less than equal to 127?

> Yeah right, I just missed that point.

Yeah, the minimum on biggest_size has nothing to do with alignment
decisions. It's just a filter to decide whether it's worth trying
to toast anything.

Having said that, I'm pretty skeptical of this patch: I think its
most likely real-world effect is going to be to waste cycles (and
create TOAST-table bloat) on the way to failing anyway. I do not
think that toasting a 20-byte field down to 18 bytes is likely to be
a productive thing to do in typical situations. The given example
looks like a cherry-picked edge case rather than a useful case to
worry about.

IOW, if I were asked to review whether the current minimum is
well-chosen, I'd be wondering if we should increase it not
decrease it.

regards, tom lane