From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
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To: | PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Why is get_actual_variable_range()'s use of SnapshotNonVacuumable safe during recovery? |
Date: | 2019-11-20 21:43:05 |
Message-ID: | CAH2-Wzn2pSqEOcBDAA40CnO82oEy-EOpE2bNh_XL_cfFoA86jw@mail.gmail.com |
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Lists: | pgsql-hackers |
I noticed that the nbtree README has an obsolete reference to the
former design of get_actual_variable_range() in the "Scans during
Recovery" section. It used to use a dirty snapshot, but doesn't
anymore. These days, get_actual_variable_range() uses
SnapshotNonVacuumable -- see commits 3ca930fc and d3751adc. I would
like to keep the README current.
My understanding is that we can trust RecentGlobalXmin to be something
useful and current during recovery, in general, so the selfuncs.c
index-only scan (which uses SnapshotNonVacuumable + RecentGlobalXmin)
can be trusted to work just as well as it would on the primary. Does
that sound correct?
The background here is that I plan on finishing off the work started
by Simon's commit 3e4b7d87; I want to *completely* remove now-dead
code that was used for "recovery pin scans". 3e4b7d87 disabled these
"pin scans" without removing them altogether, which just seems sloppy
now. There are quite a lot of comments that needlessly talk about this
pin scan mechanism in far removed places like nbtxlog.h. Also, we
waste a small amount of space in xl_btree_vacuum WAL records, since we
don't need to WAL-log lastBlockVacuumed (we also don't need to call
_bt_delitems_vacuum() one last time in the case where we don't have
anything to kill on the last block, just so the pin scan can happen --
it won't ever happen).
--
Peter Geoghegan
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