Re: Rewriting Free Space Map

Gregory Stark wrote:
> "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>
>
>> "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com> writes:
>>
>>> My original thought was to have a separate RelFileNode for each of the
>>> maps. That would require no smgr or xlog changes, and not very many
>>> changes in the buffer manager, though I guess you'd more catalog
>>> changes. You had doubts about that on the previous thread
>>> (http://archives.postgresql.org/pgsql-hackers/2007-11/msg00204.php) but
>>> the "map forks" idea certainly seems much more invasive than that.
>>>
>> The main problems with that are (a) the need to expose every type of map
>> in pg_class and (b) the need to pass all those relfilenode numbers down
>> to pretty low levels of the system. The nice thing about the fork idea
>> is that you don't need any added info to uniquely identify what relation
>> you're working on. The fork numbers would be hard-wired into whatever
>> code needed to know about particular forks. (Of course, these same
>> advantages apply to using special space in an existing file. I'm
>> just suggesting that we can keep these advantages without buying into
>> the restrictions that special space would have.)
>>
>
> One advantage of using separate relfilenodes would be that if we need to
> regenerate a map we could do it in a new relfilenode and swap it in like we do
> with heap rewrites.
>
>

Why can't you just do that with a different extension and file rename?
You'd need an exclusive lock while swapping in the new map, but you need
that anyway, IIRC, and this way you don't even need a catalog change.

cheers

andrew