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Refine the definition of page-level freezing. 

Improve comments added by commit 1de58df4 which describe the
lazy_scan_prune "freeze the page" path.  These newly revised comments
are based on suggestions from Jeff Davis.

In passing, remove nearby visibility_cutoff_xid comments left over from
commit 6daeeb1f.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Jeff Davis <pgsql@j-davis.com>
Discussion: https://postgr.es/m/ebc857107fe3edd422ef8a65191ca4a8da568b9b.camel@j-davis.com
This commit is contained in:
Peter Geoghegan 2023-01-03 10:08:55 -08:00
parent bf03cfd162
commit b37a083239
2 changed files with 15 additions and 28 deletions
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20
src/backend/access/heap/vacuumlazy.c
View File

@ -1788,13 +1788,13 @@ retry:
if (tuples_frozen == 0)
{
/*
* We're freezing all eligible tuples on the page, but have no
* freeze plans to execute. This is structured as a case where
* the page is nominally frozen so that we set pages all-frozen
* whenever no freeze plans need to be executed to make it safe.
* If this was handled via "no freeze" processing instead then
* VACUUM would senselessly waste certain opportunities to set
* pages all-frozen (not just all-visible) at no added cost.
* We have no freeze plans to execute, so there's no added cost
* from following the freeze path. That's why it was chosen.
* This is important in the case where the page only contains
* totally frozen tuples at this point (perhaps only following
* pruning). Such pages can be marked all-frozen in the VM by our
* caller, even though none of its tuples were newly frozen here
* (note that the "no freeze" path never sets pages all-frozen).
*
* We never increment the frozen_pages instrumentation counter
* here, since it only counts pages with newly frozen tuples
@ -1859,12 +1859,6 @@ retry:
if (!heap_page_is_all_visible(vacrel, buf, &cutoff, &all_frozen))
Assert(false);
/*
* It's possible that we froze tuples and made the page's XID cutoff
* (for recovery conflict purposes) FrozenTransactionId. This is okay
* because visibility_cutoff_xid will be logged by our caller in a
* moment.
*/
Assert(!TransactionIdIsValid(cutoff) ||
cutoff == prunestate->visibility_cutoff_xid);
}

23
src/include/access/heapam.h
View File

@ -145,16 +145,14 @@ typedef struct HeapPageFreeze
/*
* "Freeze" NewRelfrozenXid/NewRelminMxid trackers.
*
* Trackers used when heap_freeze_execute_prepared freezes the page, and
* when page is "nominally frozen", which happens with pages where every
* call to heap_prepare_freeze_tuple produced no usable freeze plan.
*
* "Nominal freezing" enables vacuumlazy.c's approach of setting a page
* all-frozen in the visibility map when every tuple's 'totally_frozen'
* result is true. That always works in the same way, independent of the
* need to freeze tuples, and without complicating the general rule around
* 'totally_frozen' results (which is that 'totally_frozen' results are
* only to be trusted with a page that goes on to be frozen by caller).
* Trackers used when heap_freeze_execute_prepared freezes, or when there
* are zero freeze plans for a page. It is always valid for vacuumlazy.c
* to freeze any page, by definition. This even includes pages that have
* no tuples with storage to consider in the first place. That way the
* 'totally_frozen' results from heap_prepare_freeze_tuple can always be
* used in the same way, even when no freeze plans need to be executed to
* "freeze the page". Only the "freeze" path needs to consider the need
* to set pages all-frozen in the visibility map under this scheme.
*
* When we freeze a page, we generally freeze all XIDs < OldestXmin, only
* leaving behind XIDs that are ineligible for freezing, if any. And so
@ -178,11 +176,6 @@ typedef struct HeapPageFreeze
* VACUUM scans a page that isn't cleanup locked. Both code paths are
* based on the same general idea (do less work for this page during the
* ongoing VACUUM, at the cost of having to accept older final values).
*
* When vacuumlazy.c caller decides to do "no freeze" processing, it must
* not go on to set the page all-frozen (setting the page all-visible
* could still be okay). heap_prepare_freeze_tuple's 'totally_frozen'
* results can only be used on a page that also gets frozen as instructed.
*/
TransactionId NoFreezePageRelfrozenXid;
MultiXactId NoFreezePageRelminMxid;