On Mon, 18 Jan 2021, 21:25 Álvaro Herrera, <alvhe...@alvh.no-ip.org> wrote: > > On 2021-Jan-18, Matthias van de Meent wrote: > > > Example: > > > > 1.) RI starts > > 2.) PHASE 2: filling the index: > > 2.1.) scanning the heap (live tuple is cached) > > < tuple is deleted > > < last transaction other than RI commits, only snapshot of RI exists > > < vacuum drops the tuple, and cannot remove it from the new index > > because this new index is not yet populated. > > 2.2.) sorting tuples > > 2.3.) index filled with tuples, incl. deleted tuple > > 3.) PHASE 3: wait for transactions > > 4.) PHASE 4: validate does not remove the tuple from the index, > > because it is not built to do so: it will only insert new tuples. > > Tuples that are marked for deletion are removed from the index only > > through VACUUM (and optimistic ALL_DEAD detection). > > > > According to my limited knowledge of RI, it requires VACUUM to not run > > on the table during the initial index build process (which is > > currently guaranteed through the use of a snapshot). > > VACUUM cannot run concurrently with CIC or RI in a table -- both acquire > ShareUpdateExclusiveLock, which conflicts with itself, so this cannot > occur.
Yes, you are correct. Vacuum indeed has a ShareUpdateExclusiveLock. Are there no other ways that pages are optimistically pruned? But the base case still stands, ignoring CIC snapshots in would give the semantic of all_dead to tuples that are actually still considered alive in some context, and should not yet be deleted (you're deleting data from an in-use snapshot). Any local pruning optimizations using all_dead mechanics now cannot be run on the table unless they hold an ShareUpdateExclusiveLock; though I'm unaware of any such mechanisms (other than below). > I do wonder if the problem you suggest (or something similar) can occur > via HOT pruning, though. It could not, at least not at the current HEAD, as only one tuple in a HOT-chain can be alive at one point, and all indexes point to the root of the HOT-chain, which is never HOT-pruned. See also the src/backend/access/heap/README.HOT. Regards, Matthias van de Meent
