On Thu, Mar 9, 2017 at 10:21 PM, Masahiko Sawada <sawada.m...@gmail.com> wrote: > On Wed, Mar 8, 2017 at 1:43 AM, Peter Geoghegan <p...@bowt.ie> wrote: >> On Sat, Mar 4, 2017 at 1:30 AM, Amit Kapila <amit.kapil...@gmail.com> wrote: >>>> While I can't see this explained anywhere, I'm >>>> pretty sure that that's supposed to be impossible, which this patch >>>> changes. >>>> >>> >>> What makes you think that patch will allow pg_class.relfrozenxid to be >>> advanced past opaque->btpo.xact which was previously not possible? >> >> By not reliably recycling pages in a timely manner, we won't change >> anything about the dead page. It just sticks around. This is mostly >> fine, but we still need VACUUM to be able to reason about it (to >> determine if it is in fact recyclable), which is what I'm concerned >> about breaking here. It still needs to be *possible* to recycle any >> recyclable page at some point (e.g., when we find it convenient). >> >> pg_class.relfrozenxid is InvalidTransactionId for indexes because >> indexes generally don't store XIDs. This is the one exception that I'm >> aware of, presumably justified by the fact that it's only for >> recyclable pages anyway, and those are currently *guaranteed* to get >> recycled quickly. In particular, they're guaranteed to get recycled by >> the next VACUUM. They may be recycled in the course of anti-wraparound >> VACUUM, even if VACUUM has no garbage tuples to kill (even if we only >> do lazy_cleanup_index() instead of lazy_vacuum_index()). This is the >> case that this patch proposes to have us skip touching indexes for. >> > > To prevent this, I think we need to not skip the lazy_cleanup_index > when ant-wraparound vacuum (aggressive = true) even if the number of > scanned pages is less then the threshold. This can ensure that > pg_class.relfrozenxid is not advanced past opaque->bp.xact with > minimal pain. Even if the btree dead page is leaved, the subsequent > modification makes garbage on heap and then autovauum recycles that > page while index vacuuming(lazy_index_vacuum). >
What about if somebody does manual vacuum and there are no garbage tuples to clean, won't in that case also you want to avoid skipping the lazy_cleanup_index? Another option could be to skip updating the relfrozenxid if we have skipped the index cleanup. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
