Bruno Wolff III <bruno@wolff.to> writes:
> I don't think that is precisely correct. I believe that the snapshot isn't
> set until the first query is made within the transaction.
Right. Specifically, any statement *other* than those enumerated in
PortalRunUtility() will set the snapshot. As of CVS tip (and for
several past releases IIRC), that code looks like:
/*
* Set snapshot if utility stmt needs one. Most reliable way to do this
* seems to be to enumerate those that do not need one; this is a short
* list. Transaction control, LOCK, and SET must *not* set a snapshot
* since they need to be executable at the start of a serializable
* transaction without freezing a snapshot. By extension we allow SHOW
* not to set a snapshot. The other stmts listed are just efficiency
* hacks. Beware of listing anything that can modify the database --- if,
* say, it has to update an index with expressions that invoke
* user-defined functions, then it had better have a snapshot.
*/
if (!(IsA(utilityStmt, TransactionStmt) ||
IsA(utilityStmt, LockStmt) ||
IsA(utilityStmt, VariableSetStmt) ||
IsA(utilityStmt, VariableShowStmt) ||
IsA(utilityStmt, VariableResetStmt) ||
IsA(utilityStmt, ConstraintsSetStmt) ||
/* efficiency hacks from here down */
IsA(utilityStmt, FetchStmt) ||
IsA(utilityStmt, ListenStmt) ||
IsA(utilityStmt, NotifyStmt) ||
IsA(utilityStmt, UnlistenStmt) ||
IsA(utilityStmt, CheckPointStmt)))
ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
else
ActiveSnapshot = NULL;
regards, tom lane
