Re: update on global temporary and unlogged tables

On 13/09/10 05:49, Robert Haas wrote:
> On Mon, Sep 6, 2010 at 10:55 PM, Robert Haas<robertmhaas(at)gmail(dot)com> wrote:
>> 3. With respect to unlogged tables, the major obstacle seems to be
>> figuring out a way for these to get automatically truncated at startup
>> time. As with temporary table cleanup in general, the problem here is
>> that you can't do the obvious thing of iterating through pg_class and
>> truncating each unlogged table you find without greatly complicating
>> the startup sequence. However, I think there's a fairly easy way
>> around this problem: truncating a table basically means removing all
>> segments and relation forks other than the first segment of the main
>> fork, and truncating that one to zero length. So we could do it the
>> same way we clean up temporary files - namely, based on the file name
>> - if we made the filenames for unlogged tables distinguishable from
>> those for regular and temporary tables. What I'm thinking about is
>> reserving a backend ID of -2 for this purpose via some suitable
>> constant definition, just as -1 (InvalidBackendId) represents a
>> permanent table in this context.
>
> I tried this approach and got fairly far with it, but ran into a snag
> in the buffer manager. It's fairly obvious that the buffer manager
> has to know whether a particular buffer is from an unlogged relation
> or not; for example, FlushBuffer() should skip the XLOG flush for an
> unlogged buffer, and must pass the correct backend ID to smgropen().
> So my first thought was just to define a bit BM_IS_UNLOGGED, with the
> obvious interpretation.

The LSNs on all pages in an unlogged relation should be zero, and
XLogFlush() will do nothing. That's what we rely on at the moment for
pages that are not WAL-logged for some reason, I don't think you need
any extra flag for that.

> So I went looking for bit-space in the buffer tag and quickly found
> some. ForkNumber is an enum which I suppose means a 32-bit integer,
> but we've only got three forks right now and it's hard to imagine more
> than a handful of additional ones, so what I'm tempted to do is change
> this from an enum to a 2-byte integer and replace the enum values with
> #defines. That frees up 2 bytes in the buffer tag which is more than
> plenty.

I haven't been following the discussion so I don't understand why you
need the extra bits, but no objections to reducing the fork number field.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com