Re: Proposal for CSN based snapshots

So, here's a first version of the patch. Still very much WIP.

One thorny issue came up in discussions with other hackers on this in PGCon:

When a transaction is committed asynchronously, it becomes visible to
other backends before the commit WAL record is flushed. With CSN-based
snapshots, the order that transactions become visible is always based on
the LSNs of the WAL records. This is a problem when there is a mix of
synchronous and asynchronous commits:

If transaction A commits synchronously with commit LSN 1, and
transaction B commits asynchronously with commit LSN 2, B cannot become
visible before A. And we cannot acknowledge B as committed to the client
until it's visible to other transactions. That means that B will have to
wait for A's commit record to be flushed to disk, before it can return,
even though it was an asynchronous commit.

I personally think that's annoying, but we can live with it. The most
common usage of synchronous_commit=off is to run a lot of transactions
in that mode, setting it in postgresql.conf. And it wouldn't completely
defeat the purpose of mixing synchronous and asynchronous commits
either: an asynchronous commit still only needs to wait for any
already-logged synchronous commits to be flushed to disk, not the commit
record of the asynchronous transaction itself.

Ants' original design with a separate commit-sequence-number that's
different from the commit LSN would not have this problem, because that
would allow the commits to become visible to others in out-of-WAL-order.
However, the WAL order == commit order is a nice and simple property,
with other advantages.

Some bigger TODO items:

* Logical decoding is broken. I hacked on it enough that it looks
roughly sane and it compiles, but didn't spend more time to debug.

* I expanded pg_clog to 64-bits per XID, but people suggested keeping
pg_clog as is, with two bits per commit, and adding a new SLRU for the
commit LSNs beside it. Probably will need to do something like that to
avoid bloating the clog.

* Add some kind of backend-private caching of clog, to make it faster to
access. The visibility checks are now hitting the clog a lot more
heavily than before, as you need to check the clog even if the hint bits
are set, if the XID falls between xmin and xmax of the snapshot.

* Transactions currently become visible immediately when a WAL record is
inserted, before it's flushed. That's wrong, but shouldn't be difficult
to fix (except for the async commit issue explained above).

- Heikki