"Tom Lane" <[EMAIL PROTECTED]> writes:

>>> set_current_snapshot() would have to sanity check that the xmin of the new
>>> snapshot isn't older than the current globaloldestxmin. 
>> That would solve the backend to backend IPC problem nicely.
> But it fails on the count of making sure that globaloldestxmin doesn't
> advance past the snap you want to use.  And exactly how will you pass
> a snap through a table?  It won't become visible until you commit ...
> whereupon your own xmin isn't blocking the advance of globaloldestxmin.

Hm, good point. You could always do it in a separate connection, but that
starts to get annoying. I was more envisioning passing it around out-of-band
though, something like:

$snap = $db->execute(select current_snapshot());

  for each db {
      if (fork())
          $slave[i] = $db->connect();
          $slave[i]->execute(select set_snapshot($snap));
          $slave[i]->execute(copy table[i] to file[i]);

I'm also wondering about something like:

  $snap = $db->execute(select current_snapshot());

  if (fork())
      $slave = $db->connect();
      $slave->execute(select set_snapshot($snap);
      $slave->execute(copy tab from hugefile);
      signal parent
  } else {
      while(no signal yet) {
          $rows_loaded_so_far = $db->execute(select count(*) from tab);

Sorry for the vaguely perlish pseudocode but it's the clearest way I can think
to write it. I don't think it would make much sense to try to do anything like
this in plpgsql; I think you really do want to be doing it in a language
outside the database where it's easier to open multiple connections and handle

I realize the second idea might take more hackery than just setting the
snapshot... In particular as written above it wouldn't work because the slave
would be writing with a new xid that isn't actually in the snapshot.

  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com

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