On Fri, Dec 7, 2018 at 11:34 PM Alvaro Herrera <alvhe...@2ndquadrant.com> wrote: > On 2018-Dec-07, Robert Haas wrote: > > Full disclosure: EDB has a feature like this and has for years, but it > > uses a subtransaction per statement, not a subtransaction per row. > > Well, this implementation only uses one subtransaction per statement; > Andres says per-row referring to the case of one INSERT per row, so it's > still one statement.
I'd like to add my 2 cents. I think usage of subtransaction per statement for statement level rollback is fair. It's unlikely we're going to invent something better in this part. The issue here is that our subtransaction mechanism is not effective enough to use it for every statement. Especially when there are many statements and each of them touches the only row. But we can improve that. The first thing, which comes to the mind is undo log. When you have undo log, then on new subtransaction you basically memorize the current undo log position. If subtransaction rollbacks, you have to just play back undo log until reach memorized position. This might be an option for zheap. But actually, I didn't inspect zheap code to understand how far we're from this. But nevetheless, we also have to do something with our heap, which is not undo-based storage. I think it is possible to implement a compromise solution, which allows to accelerate small subtransactions on heap. Instead of allocating xid for subtransaction immediately on write, we can start with using top transaction xid. But also allocate small memory buffer to memorize locations of heap tuples added or modified by this subtransaction. If we wouldn't overflow this small buffer, we can use it to rollback state of heap on subtransaction abort. Otherwise, we can decide to nevertheless allocate xid for this subtransaction. But then xid allocation would happen only for large-enough subtransactions. If there is an interest in this, I can write a POC. ------ Alexander Korotkov Postgres Professional: http://www.postgrespro.com The Russian Postgres Company
