On 2022-Sep-22, Simon Riggs wrote: > On Mon, 19 Sept 2022 at 00:16, Greg Stark <st...@mit.edu> wrote:
> > VACUUM was willing to remove a committed-dead tuple immediately if it > > was > > deleted by the same transaction that inserted it. The idea is that > > such a > > tuple could never have been visible to any other transaction, so we > > don't > > need to keep it around to satisfy MVCC snapshots. However, there was > > already an exception for tuples that are part of an update chain, and > > this > > exception created a problem: we might remove TOAST tuples (which are > > never > > part of an update chain) while their parent tuple stayed around (if it > > was > > part of an update chain). This didn't pose a problem for most things, > > since the parent tuple is indeed dead: no snapshot will ever consider it > > visible. But MVCC-safe CLUSTER had a problem, since it will try to copy > > RECENTLY_DEAD tuples to the new table. It then has to copy their TOAST > > data too, and would fail if VACUUM had already removed the toast tuples. > Good research Greg, thank you. Only took 10 years for me to notice it > was gone ;-) But this begs the question: is the proposed change safe, given that ancient consideration? I don't think TOAST issues have been mentioned in this thread so far, so I wonder if there is a test case that verifies that this problem doesn't occur for some reason. -- Álvaro Herrera PostgreSQL Developer — https://www.EnterpriseDB.com/
