On Mon, Feb 17, 2025 at 12:11 PM Masahiko Sawada <sawada.m...@gmail.com> wrote:
> > If the idea is to never allow parallelism in vacuum, then I think > > disabling eager scanning during manual parallel vacuum seems > > reasonable. People could use vacuum freeze if they want more freezing. > > IIUC the purpose of parallel vacuum is incompatible with the purpose > of auto vacuum. The former is aimed to execute the vacuum as fast as > possible using more resources, whereas the latter is aimed to execute > the vacuum while not affecting foreground transaction processing. It's > probably worth considering to enable parallel vacuum even for > autovacuum in a wraparound situation, but the purpose would remain the > same. > That's assuming that the database is running with autovacuum_cost_delay > 0. There's presumably some number of systems that intentionally do not throttle autovacuum. Another consideration is that people are free to set vacuum_cost_page_miss = 0; in those cases it would still be useful to parallelize at least phase 1 and 2. Of course, folks can also set vacuum_cost_page_dirty = 0, in which case parallelization would help phase 2 and 3. In terms of less hypothetical scenarios, I've definitely run into cases where 1 table accounts for 90%+ of the space used by a database. Parallelism would help ensure the single table is still processed in a timely fashion. (Assuming non-default settings for vacuum throttling.)
