Re: unlogged tables vs. GIST

On Fri, Dec 17, 2010 at 3:03 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
>> On 17.12.2010 21:32, Robert Haas wrote:
>>> I guess the question is whether it's right to conflate "table is
>>> unlogged" with "LSN is fake".  It's not immediately obvious to me that
>>> those concepts are isomorphic, although though the reverse isn't
>>> obvious to me either.
>
>> The buffer manager only needs to know if it has to flush the WAL before
>> writing the page to disk. The flag just means that the buffer manager
>> never needs to do that for this buffer. You're still free to store a
>> real LSN there if you want to, it just won't cause any WAL flushes.
>
> Yeah.  I think that BM_UNLOGGED might be a poor choice for the flag name,
> just because it overstates what the bufmgr needs to assume.  It might be
> better to reverse the flag sense, and have a new flag that *is* set if
> the page contains an LSN that we have to check against WAL.

I was actually thinking of adding BM_UNLOGGED even before this
discussion, because that would allow unlogged buffers to be excluded
from non-shutdown checkpoints. We could add two flags with different
semantics that take on, under present rules, the same value, but I'd
be disinclined to burn the extra bit without a concrete need.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company