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

Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On 17 April 2012 17:22, Jameison Martin <jameisonb(at)yahoo(dot)com> wrote:
>> The following patch truncates trailing null attributes from heap rows to
>> reduce the size of the row bitmap.

> This is an interesting patch, but its has had various comments made about it.

> When I look at this I see that it would change the NULL bitmap for all
> existing rows, which means it forces a complete unload/reload of data.

Huh? I thought it would only change how *new* tuples were stored.
Old tuples ought to continue to work fine.

I'm not really convinced that it's a good idea in the larger scheme
of things --- your point in a nearby thread that micro-optimizing
storage space at the expense of all else is not good engineering
applies here. But I don't see that it forces data reload. Or if
it does, that should be easily fixable.

> ... Have another flag which indicates
> when a partial trailing col trimmed NULL bitmap is in use.

That might be useful for forensic purposes, but on the whole I suspect
it's just added complexity (and eating up a valuable infomask bit)
for relatively little gain.

> ... decide whether a table will benefit from full or partial bitmap and
> set that in the tupledesc. That way the tupledesc will show
> heap_form_tuple which kind of null bitmap is preferred for new tuples.
> That preference might be settable by user on or off, but the default
> would be for postgres to decide that for us based upon null stats etc,
> which we would decide at ANALYZE time.

And that seems like huge overcomplication. I think we could probably
do fine with some very simple fixed policy, like "don't bother with
this for tables of less than N columns", where N is maybe 64 or so
and chosen to match the MAXALIGN boundary where there actually could
be some savings from trimming the null bitmap.

(Note: I've not read the patch, so maybe Jameison already did something
of the sort.)

regards, tom lane