Phase 2
=======

Idea is that we keep a transaction running on primary with a snapshot
that prevents VACUUM from ever removing records that a query on the
standby can see. So there are never any query conflicts.

Make two connections to primary, P1 and P2. One to Standby, S.

P1:
BEGIN;
SELECT txid_current(); INTO Pn->xid
SELECT pg_sleep(10000000000);

P2:
BEGIN;
SELECT txid_current(); INTO Pn->xid
SELECT pg_sleep(10000000000);

Then every 1sec

S: GetOldestXmin() INTO S->xmin

/*
 * If both P1 and P2 are older than oldest xmin on S
 * then we cancel the oldest of P1 and P2, and
 * start a new xid on that channel. So that P1 and P2
 * slowly step forwards, as queries complete on the
 * standby.
 */
if (TransactionIdPrecedes(P1->xid, S->xmin) &&
   TransactionIdPrecedes(P2->xid, S->xmin))
{
  if (TransactionIdPrecedes(P1->xid, P2->xid))
       cancel_and_rerun(1)
  else
       cancel_and_rerun(2)
)

cancel_and_rerun()
{
  cancel Pn

BEGIN;
SELECT txid_current(); INTO Pn->xid
SELECT pg_sleep(10000000000);
}