Re: making relfilenodes 56 bits

Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)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.

* Note: various places use RelFileLocator in hashtable keys. Therefore,
* there *must not* be any unused padding bytes in this struct. That
* should be safe as long as all the fields are of type Oid.
*/
typedef struct RelFileLocator
{
Oid spcOid; /* tablespace */
Oid dbOid; /* database */
RelFileNumber relNumber; /* relation */
} RelFileLocator;

regards, tom lane