Simon Riggs wrote:
If you don't mark subtrans, you will need to keep things in shared memory. I agree you can defer marking subtrans until your shared memory fills, but you will still need to keep track of the parent for each subtransaction until you do. If you don't *know* the parent, how will you do this? You haven't explained how you will write the relationship between a subtransaction and its parent into WAL, for each subtransaction and for an arbitrarily large number of subtransactions.
I was thinking that it's done in AssignTransactionId(). Similarly to what you're doing in your patch, but instead of just one xid with it's parent, a list of (xid, parent) pairs would be stored in the WAL record.
Upon replaying that WAL record, those parent-child relationships are recorded in subtrans, and the child xids are removed from UnobservedXids.
We seem to be scrabbling around looking for a second solution. Even if we find one you'll need to show that its substantially better than the one that's already on the table. But right now it doesn't seem like we're on the verge of a radically better approach, for this part.
I'm trying to tease apart the important aspects of the design, and trying to reduce it into the simplest thing that can work. For example, since it seems like we need UnobservedXids array anyway, can we use just that, and not need the PROC entries? And if we need to sometimes WAL log in AssignTransactionId(), could we use just that one mechanism for transferring parent-child relationships, and not need the extra field in all WAL records? The less mechanisms there is to do stuff, the simpler the patch, the less codepaths to test, and so forth.
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