Re: init_sequence spill to hash table

On Fri, Nov 15, 2013 at 3:23 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> > I think it'd be a better idea to integrate the sequence caching logic
> > into the relcache. There's a comment about it:
> > * (We can't
> > * rely on the relcache, since it's only, well, a cache, and may decide
> to
> > * discard entries.)
> > but that's not really accurate anymore. We have the infrastructure for
> > keeping values across resets and we don't discard entries.
>
> We most certainly *do* discard entries, if they're not open when a cache
> flush event comes along.
>
> I suppose it'd be possible to mark a relcache entry for a sequence
> as locked-in-core, but that doesn't attract me at all. A relcache
> entry is a whole lot larger than the amount of state we really need
> to keep for a sequence.
>
> One idea is to have a hashtable for the sequence-specific data,
> but to add a link field to the relcache entry that points to the
> non-flushable sequence hashtable entry. That would save the second
> hashtable lookup as long as the relcache entry hadn't been flushed
> since last use, while not requiring any violence to the lifespan
> semantics of relcache entries. (Actually, if we did that, it might
> not even be worth converting the list to a hashtable? Searches would
> become a lot less frequent.)
>
>
Unless I've misunderstood something it looks like this would mean giving
heamam.c and relcache.c knowledge of sequences.
Currently relation_open is called from open_share_lock in sequence.c. The
only way I can see to do this would be to add something like
relation_open_sequence() in heapam.c which means we'd need to invent
RelationIdGetSequenceRelation() and use that instead
of RelationIdGetRelation() and somewhere along the line have it pass back
the SeqTableData struct which would be tagged onto RelIdCacheEnt.

I think it can be done but I don't think it will look pretty.
Perhaps if there was a more generic way... Would tagging some void
*rd_extra only the RelationData be a better way? And just have sequence.c
make use of that for storing the SeqTableData.

Also I'm wondering what we'd do with all these pointers when someone does
DISCARD SEQUENCES; would we have to invalidate the relcache or would it
just be matter of looping over it and freeing of the sequence data setting
the pointers to NULL?

Regards

David Rowley