Re: Nested transactions: low level stuff

Tom Lane wrote:
> Hiroshi Inoue <Inoue(at)tpf(dot)co(dot)jp> writes:
> > Bruce Momjian wrote:
> >> You mean abort subtransactions? Each subtransaction gets its own
> >> transaction id, so we just mark that as aborted --- there is no undo of
> >> tuples, though I had originally suggested that approach years ago.
>
> > Vadim planned to implement the savepoints functionality
> > using UNDO mechanism. AFAIR it was never denied explicitly.
>
> Given all the flak we got about WAL growth during the time we had that
> code enabled, I think there's no chance that UNDO will be the preferred
> path. It's not workable with big transactions.
>
> There are other problems besides WAL bloat, too. I realized while I was
> working on the btree code a few weeks ago that it's fundamentally
> unfriendly to UNDO, because there are some operations you'd want to
> UNDO (viz, insertion of a leaf item pointing at a heap tuple) and some
> you would not (viz, splitting of index pages and subsequent insertion of
> items into upper tree levels). But the same WAL entry might include
> both kinds of operation. This could be got round, perhaps, but that
> code is overcomplicated already ...

I assumed the UNDO would have had to be in a separate place, or allow
compression of the WAL file to keep needed UNDO stuff but get rid of
unneeded stuff --- it was all quite complicated.

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