On Jan7, 2011, at 12:41 , Joachim Wieland wrote: > On Thu, Dec 30, 2010 at 7:31 AM, Joachim Wieland <j...@mcknight.de> wrote: > These are the implementation details and restrictions of the patch: > > The exporting transaction > > - should be read-only (to discourage people from expecting that writes of > the exporting transaction can be seen by the importing transaction)
That seems overly strict. Wouldn't a warning about this in the docs suffice? > - must not be a subtransaction (we don't add subxips of our own transaction > to the snapshot, so importing the snapshot later would result in missing > subxips) If that is necessary, wouldn't a sub-transaction that was sub-committed before exporting the snapshot also pose a problem? The snapshot wouldn't include the sub-transaction's xid, so once the exporting transaction has committed any importing transaction would suddenly see the sub-transaction's changes. > - adds its own xid (if any) to the xip-array Per the previous paragraph, I believe it should also add the xid's of all non-aborted sub-transactions to the subxip-array, overflowing the array if necessary. > The importing transaction > > - will not import a snapshot of the same backend (even though it would > probably work) > - will not import a snapshot of a different database in the cluster > - should be isolation level serializable > - must not be a subtransaction (can we completely rollback on > subxact-rollback?) Sounds fine so far. > - leaves curcid as is, otherwise effects of previous commands would get > lost > and magically appear later when curcid increases > - applies xmin, xmax, xip, subxip values of the exported snapshot to > GetActiveSnapshot() and GetTransactionSnapshot() > - takes itself out of the xip array > - updates MyProc->xmin but sets it only backwards but not forwards Change a snapshot mid-transaction like that seems risky. I'd suggest to simply decree that an importing transaction must be read-only and must not yet have set its snapshot. So effectively, the only allowed calling sequence would be begin; set transaction isolation level serializable read only; pg_import_snapshot(...); best regards, Florian Pflug -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
