Re: unlogged tables vs. GIST

On 15.01.2013 20:33, Robert Haas wrote:
> On Tue, Jan 15, 2013 at 1:10 PM, Heikki Linnakangas
>> Could we stash the counter e.g. in the root page of the index?
>
> That would require maintaining a counter per table rather than a
> single global counter, which would be bad because then we'd need to
> store one counter in shared memory for every table, rather than just
> one, period, which runs up against the fixed sizing of shared memory.

I was thinking of just adding a new field to the root page header, and
use that field as the counter. Something like:

XLogRecPtr
GetXLogRecPtrForTemp(void)
{
rootbuf = ReadBuffer(rel, GIST_ROOT_BLKNO);
opaq = GistPageGetOpaque(BufferGetPage(rootbuf));

LockBuffer(rootbuf, GIST_EXCLUSIVE);
nsn = opaq->counter++
UnlockReleaseBuffer(rootbuf)
return nsn;
}

or perhaps we need to use locking mechanism for that, like just a new
global lwlock or spinlock, to avoid deadlocks if someone is just
splitting the root page. In any case, the fixed-sizedness of shared
memory isn't an issue here.

- Heikki