Re: making relfilenodes 56 bits

On Sat, Jul 30, 2022 at 1:35 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> > On 2022-Jul-29, Robert Haas wrote:
> >> Yeah, if we think it's OK to pass around structs, then that seems like
> >> the right solution. Otherwise functions that take RelFileLocator
> >> should be changed to take const RelFileLocator * and we should adjust
> >> elsewhere accordingly.
>
> > We do that in other places.  See get_object_address() for another
> > example.  Now, I don't see *why* they do it.
>
> If it's a big struct then avoiding copying it is good; but RelFileLocator
> isn't that big.
>
> While researching that statement I did happen to notice that no one has
> bothered to update the comment immediately above struct RelFileLocator,
> and it is something that absolutely does require attention if there
> are plans to make RelFileNumber something other than 32 bits.

I think we need to update this comment in the patch where we are
making RelFileNumber 64 bits wide.  But as such I do not see a problem
in using RelFileLocator directly as key because if we make
RelFileNumber 64 bits then its structure will already be 8 byte
aligned so there should not be any padding.  However, if we use some
other structure as key which contain RelFileLocator i.e.
RelFileLocatorBackend then there will be a problem.  So for handling
that issue while computing the key size (wherever we have
RelFileLocatorBackend as key) I have avoided the padding bytes in size
by introducing this new macro[1].

[1]
#define SizeOfRelFileLocatorBackend \
(offsetof(RelFileLocatorBackend, backend) + sizeof(BackendId))

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com