Re: [PATCH] backend: compare word-at-a-time in bcTruelen

On Tue, Jun 16, 2009 at 1:03 PM, Robert Haas<robertmhaas(at)gmail(dot)com> wrote:
>
>> On the flip side, I am curious as to if the arguments to a stored
>> procedure are always aligned or not.  Never had a case to care before,
>> but if palloc() is always going to return an aligned chunk of memory
>> (per MemSetAligned in c.h) it makes me wonder.
>
> Well, if it's char(n) for n <~ 126, it's going to have a 1-byte
> varlena header...

There are two points here that kind of cancel each other out :)

If the data is in fact returned from a palloc because it was the
result of some other function call then it will almost certainly have
a 4-byte header and that'll be aligned. There are some exceptions
where functions are just returning copies and copy the whole datum
though, but the point is we normally don't toast or pack varlenas
unless they're being stored on disk.

However that's all irrelevant because there's no guarantee the data
being passed will have been palloced at all. You could get a pointer
to data in a shared buffer. Ie, data on disk. That will be aligned
based on how tuples are packed on disk which is precisely where we go
out of our way to avoid wasting space on alignment.

--
greg
http://mit.edu/~gsstark/resume.pdf