txid - 8 byte transaction ID's
==============================

Based on xxid module from Slony-I.  

The goal is to make PostgreSQL internal transaction ID and snapshot
data usable externally.  They cannot be used directly as the
internal 4-byte value wraps around and thus breaks indexing.

This module extends the internal value with wraparound cound (epoch).
It uses relaxed method for wraparound check.  There is a table
txid.epoch (epoch, last_value) which is used to check if the xid
is in current, next or previous epoch.  It requires only occasional
read-write access - ca. after 100k - 500k transactions.

Also it contains type 'txid_snapshot' and following functions:


get_current_txid() returns int8

  Current transaction ID

get_current_snapshot() returns txid_snapshot

  Current snapshot

get_snapshot_xmin( [snap] ) returns int8      -- 

  Smallest TXID in snapshot.  TXID's smaller than this
  are all visible in snapshot.  Without argument uses
  current snapshot.

get_snapshot_xmax( [snap] ) returns int8

  Largest TXID in snapshot.  TXID's starting from this one are
  all invisible in snapshot.  Without argument uses current snapshot.
	
get_snapshot_values( [snap] ) setof int8

  List of uncommitted TXID's in snapshot, that are invisible
  in snapshot.  Values are between xmin and xmax.  Without
  argument uses current snapshot.

txid_in_snapshot(id, snap) returns bool

  Is TXID visible in snapshot?

txid_not_in_snapshot(id, snap) returns bool

  Is TXID invisible in snapshot?


Problems
--------

- it breaks when there are more than 2G tx'es between calls.
  this can only be solved by moving it into PostgreSQL core.
 
- functions that create new txid's should be 'security definers'
  thus better protecting txid_epoch table.

- After loading database from backup you should do:

  UPDATE txid.epoch SET epoch = epoch + 1,
                        last_value = (get_current_txid() & 0xFFFFFFFF);

  Again, if the txid could be in core, the support could be
  integraded into pg_dump that could dump function call:

    select txid_set_epoch(old_epoch, old_xid)

  that bumps current epoch if needed, thus guaranteeing that
  new txids will be greater than old ones.