Re: Sequence Access Method WIP

On 2013-11-15 20:08:30 +0200, Heikki Linnakangas wrote:
> It's pretty hard to review the this without seeing the "other"
> implementation you're envisioning to use this API. But I'll try:

We've written a distributed sequence implementation against it, so it
works at least for that use case.

While certainly not release worthy, it still might be interesting to
look at.
http://git.postgresql.org/gitweb/?p=users/andresfreund/postgres.git;a=blob;f=contrib/bdr/bdr_seq.c;h=c9480c8021882f888e35080f0e7a7494af37ae27;hb=bdr

bdr_sequencer_fill_sequence pre-allocates ranges of values,
bdr_sequence_alloc returns them upon nextval().

> I wonder if the level of abstraction is right. The patch assumes that the
> on-disk storage of all sequences is the same, so the access method can't
> change that. But then it leaves the details of actually updating the on-disk
> block, WAL-logging and all, as the responsibility of the access method.
> Surely that's going to look identical in all the seqams, if they all use the
> same on-disk format. That also means that the sequence access methods can't
> be implemented as plugins, as plugins can't do WAL-logging.

Well, it exposes log_sequence_tuple() - together with the added "am
private" column of pg_squence that allows to do quite a bit of different
things. I think unless we really implement pluggable RMGRs - which I
don't really see coming - we cannot be radically different.

> The comment in seqam.c says that there's a private column reserved for
> access method-specific data, called am_data, but that seems to be the only
> mention of am_data in the patch.

It's amdata, not am_data :/. Directly at the end of pg_sequence.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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