On Thu, May 11, 2006 at 12:18:06PM -0500, Jim C. Nasby wrote: > > Yes, because there can be more than one active snapshot within a single > > transaction (think about volatile functions in particular). > > Any documentation on how snapshot's work? They're a big mystery to me. > :( A snapshot is a particular view on a database. In particular, you have to be able to view a version of the database that doesn't have you own changes, otherwise an UPDATE would keep updating the same tuple. Also, for example, a cursor might see an older version of the database than queries being run. I don't know of any particular information about it though. Google wasn't that helpful. > > No; you forgot about subtransactions. > > Oh, I thought those were done with cmin and cmax... if that's not what > cmin/cmax are for, then what is? cmin/cmax are command counters. So in the sequence: BEGIN; SELECT 1; SELECT 2; The second query runs as the same transaction ID but a higher command ID so it can see the result of the previous query. Subtransactions are (AIUI anyway) done by having transactions depend on other transactions. When you start a savepoint you start a new transaction ID whose status is tied to its top-level transaction ID but can also be individually rolledback. Have a nice day, -- Martijn van Oosterhout <kleptog@xxxxxxxxx> http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to litigate.
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