pgsql: vacuumlazy.c: document vistest and OldestXmin.

vacuumlazy.c: document vistest and OldestXmin.

Explain the relationship between vacuumlazy.c's vistest and OldestXmin
cutoffs. These closely related cutoffs are different in subtle but
important ways. Also document a closely related rule: we must establish
rel_pages _after_ OldestXmin to ensure that no XID < OldestXmin can be
missed by lazy_scan_heap().

It's easier to explain these issues by initializing everything together,
so consolidate initialization of vacrel state. Now almost every vacrel
field is initialized by heap_vacuum_rel(). The only remaining exception
is the dead_items array, which is still managed by lazy_scan_heap() due
to interactions with how we initialize parallel VACUUM.

Also move the process that updates pg_class entries for each index into
heap_vacuum_rel(), and adjust related assertions. All pg_class updates
now take place after lazy_scan_heap() returns, which seems clearer.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Reviewed-By: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/20211211045710.ljtuu4gfloh754rs@alap3.anarazel.de
Discussion: https://postgr.es/m/CAH2-WznYsUxVT156rCQ+q=YD4S4=1M37hWvvHLz-H1pwSM8-Ew@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/73f6ec3d3c8d5786c54373e71a096e5acf78e7ca

Modified Files
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src/backend/access/heap/vacuumlazy.c | 173 +++++++++++++++++++----------------
1 file changed, 92 insertions(+), 81 deletions(-)