(CC'ed to Melanie) On Fri, May 2, 2025 at 11:59 AM Masahiko Sawada <sawada.m...@gmail.com> wrote: > > Hi, > > I hit the assertion failure in the subject line with the following script: > > create table t (a int) with (autovacuum_enabled = off); > insert into t select generate_series(1, 1_000_000); > vacuum t; > insert into t select generate_series(1, 1_000_000); > set vacuum_freeze_min_age to 0; > vacuum t; > > When the success count reaches to 0, we disable the eager scan by > resetting related fields as follows: > > /* > * If we hit our success cap, permanently disable eager > * scanning by setting the other eager scan management > * fields to their disabled values. > */ > vacrel->eager_scan_remaining_fails = 0; > vacrel->next_eager_scan_region_start = InvalidBlockNumber; > vacrel->eager_scan_max_fails_per_region = 0; > > However, there is a possibility that we have already eagerly scanned > another page and returned it to the read stream before we freeze the > eagerly-scanned page and disable the eager scan. In this case, the > next block that we retrieved from the read stream could also be an > eagerly-scanned page.
I've added it to Open Items for v18. If I understand correctly, there's a potential scenario where we might eagerly scan more pages than permitted by the success and failure caps. One question is: what is the practical magnitude of this excess scanning? If the overflow could be substantial (for instance, eagerly scanning 30% of table pages), we should consider revising our eager scanning mechanism. One potential solution would be to implement a counter tracking the number of eagerly scanned but unprocessed pages. This counter could then inform the decision-making process in find_next_unskippable_block() regarding whether to proceed with eager scanning of additional pages. Alternatively, if the excess scanning proves negligible in practice, we could adopt a simpler solution by allowing vacrel->eager_scan_remaining_successes to accept negative values and removing the assertion in question. Regards, -- Masahiko Sawada Amazon Web Services: https://aws.amazon.com
