Alvaro Herrera <[email protected]> wrote: > On 2026-Mar-16, Matthias van de Meent wrote: > > > On Mon, 16 Mar 2026 at 21:15, Antonin Houska <[email protected]> wrote: > > > > Anyway (fortunately?), the concurrent use of slots by REPACK is limited > > > because, during the initialization of logical decoding, the backend needs > > > to > > > wait for all the transactions having XID assigned to finish, and these > > > include > > > the already running REPACK commands. See SnapBuildWaitSnapshot() and > > > callers > > > if you're interested in details. > > > > Huh, so would you be able to run more than one Repack Concurrently in > > the same database? ISTM that would not be possible, apart from > > possibly a mechanism comparable to the SAFE_IN_IC flag (to not wait on > > those backends). > > Yeah, this sounds kind of bad news ...
Admittedly, it is a problem. I tried to address this in pg_squeeze by pre-allocating slots when it's clear (due to scheduling) that more than one table needs to be processed. This was an effort to achieve the best possible performance rather than a response to complaints of users about low throughput. Nevertheless, I'm glad I happened to mention it before it's too late. Regarding solution, a flag like SAFE_IN_IC alone does not help. The information that particular transaction is used by REPACK (and therefore it does not have to be decoded) would need to be propagated to the xl_running_xacts WAL record too. The enhancements I wrote for PG 20 (not all of them posted yet) that aim at eliminating the impact of REPACK on VACUUM xmin horizon should fix this problem: due to the MVCC-safety (i.e. preserving xmin/xmax of the tuples), REPACK will not need XID assigned (except for catalog changes, which will happen in separate transactions), so it won't block the logical decoding setup of other backends. So the question is whether we should implement a workaround for PG 19, that won't be needed in v20. -- Antonin Houska Web: https://www.cybertec-postgresql.com
