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Update PruneState.all_[visible|frozen] earlier in pruning

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During pruning and freezing in phase I of vacuum, we delay clearing
all_visible and all_frozen in the presence of dead items. This allows
opportunistic freezing if the page would otherwise be fully frozen,
since those dead items are later removed in vacuum phase III.

To move the VM update into the same WAL record that
prunes and freezes tuples, we must know whether the page will
be marked all-visible/all-frozen before emitting WAL. Previously we
waited until after emitting WAL to update all_visible/all_frozen to
their correct values.

The only barrier to updating these flags immediately after deciding
whether to opportunistically freeze was that while emitting WAL for a
record freezing tuples, we use the pre-corrected value of all_frozen to
compute the snapshot conflict horizon. By determining the conflict
horizon earlier, we can update the flags immediately after making the
opportunistic freeze decision.

This is required to set the VM in the XLOG_HEAP2_PRUNE_VACUUM_SCAN
record emitted by pruning and freezing.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_ZMw6Npd_qm2KM%2BFwQ3cMOMx1Dh3VMhp8-V7SOLxdK9-g%40mail.gmail.com
This commit is contained in:
Melanie Plageman
2025-11-20 11:40:36 -05:00
parent 351d7e2441
commit 65ec565b19
1 changed files with 59 additions and 58 deletions
Download Patch File Download Diff File Expand all files Collapse all files

117
src/backend/access/heap/pruneheap.c
View File

@@ -129,6 +129,13 @@ typedef struct
int lpdead_items; /* number of items in the array */
OffsetNumber *deadoffsets; /* points directly to presult->deadoffsets */
/*
* The snapshot conflict horizon used when freezing tuples. The final
* snapshot conflict horizon for the record may be newer if pruning
* removes newer transaction IDs.
*/
TransactionId frz_conflict_horizon;
/*
* all_visible and all_frozen indicate if the all-visible and all-frozen
* bits in the visibility map can be set for this page after pruning.
@@ -138,11 +145,11 @@ typedef struct
* bits. It is only valid if we froze some tuples, and all_frozen is
* true.
*
* NOTE: all_visible and all_frozen don't include LP_DEAD items. That's
* convenient for heap_page_prune_and_freeze(), to use them to decide
* whether to freeze the page or not. The all_visible and all_frozen
* values returned to the caller are adjusted to include LP_DEAD items at
* the end.
* NOTE: all_visible and all_frozen initially don't include LP_DEAD items.
* That's convenient for heap_page_prune_and_freeze() to use them to
* decide whether to freeze the page or not. The all_visible and
* all_frozen values returned to the caller are adjusted to include
* LP_DEAD items after we determine whether to opportunistically freeze.
*/
bool all_visible;
bool all_frozen;
@@ -388,6 +395,22 @@ heap_page_will_freeze(Relation relation, Buffer buffer,
* critical section.
*/
heap_pre_freeze_checks(buffer, prstate->frozen, prstate->nfrozen);
/*
* Calculate what the snapshot conflict horizon should be for a record
* freezing tuples. We can use the visibility_cutoff_xid as our cutoff
* for conflicts when the whole page is eligible to become all-frozen
* in the VM once we're done with it. Otherwise, we generate a
* conservative cutoff by stepping back from OldestXmin.
*/
if (prstate->all_frozen)
prstate->frz_conflict_horizon = prstate->visibility_cutoff_xid;
else
{
/* Avoids false conflicts when hot_standby_feedback in use */
prstate->frz_conflict_horizon = prstate->cutoffs->OldestXmin;
TransactionIdRetreat(prstate->frz_conflict_horizon);
}
}
else if (prstate->nfrozen > 0)
{
@@ -433,10 +456,10 @@ heap_page_will_freeze(Relation relation, Buffer buffer,
* now. The 'params.cutoffs', 'presult', 'new_relfrozen_xid' and
* 'new_relmin_mxid' arguments are required when freezing. When
* HEAP_PAGE_PRUNE_FREEZE option is passed, we also set presult->all_visible
* and presult->all_frozen on exit, to indicate if the VM bits can be set.
* They are always set to false when the HEAP_PAGE_PRUNE_FREEZE option is not
* passed, because at the moment only callers that also freeze need that
* information.
* and presult->all_frozen after determining whether or not to
* opporunistically freeze, to indicate if the VM bits can be set. They are
* always set to false when the HEAP_PAGE_PRUNE_FREEZE option is not passed,
* because at the moment only callers that also freeze need that information.
*
* presult contains output parameters needed by callers, such as the number of
* tuples removed and the offsets of dead items on the page after pruning.
@@ -522,6 +545,7 @@ heap_page_prune_and_freeze(PruneFreezeParams *params,
prstate.hastup = false;
prstate.lpdead_items = 0;
prstate.deadoffsets = presult->deadoffsets;
prstate.frz_conflict_horizon = InvalidTransactionId;
/*
* Caller may update the VM after we're done. We can keep track of
@@ -541,10 +565,11 @@ heap_page_prune_and_freeze(PruneFreezeParams *params,
* are tuples present that are not visible to everyone or if there are
* dead tuples which are not yet removable. However, dead tuples which
* will be removed by the end of vacuuming should not preclude us from
* opportunistically freezing. Because of that, we do not clear
* all_visible and all_frozen when we see LP_DEAD items. We fix that at
* the end of the function, when we return the value to the caller, so
* that the caller doesn't set the VM bits incorrectly.
* opportunistically freezing. Because of that, we do not immediately
* clear all_visible and all_frozen when we see LP_DEAD items. We fix
* that after scanning the line pointers. We must correct all_visible and
* all_frozen before we return them to the caller, so that the caller
* doesn't set the VM bits incorrectly.
*/
if (prstate.attempt_freeze)
{
@@ -781,6 +806,22 @@ heap_page_prune_and_freeze(PruneFreezeParams *params,
do_hint_prune,
&prstate);
/*
* While scanning the line pointers, we did not clear
* all_visible/all_frozen when encountering LP_DEAD items because we
* wanted the decision whether or not to freeze the page to be unaffected
* by the short-term presence of LP_DEAD items. These LP_DEAD items are
* effectively assumed to be LP_UNUSED items in the making. It doesn't
* matter which vacuum heap pass (initial pass or final pass) ends up
* setting the page all-frozen, as long as the ongoing VACUUM does it.
*
* Now that we finished determining whether or not to freeze the page,
* update all_visible and all_frozen so that they reflect the true state
* of the page for setting PD_ALL_VISIBLE and VM bits.
*/
if (prstate.lpdead_items > 0)
prstate.all_visible = prstate.all_frozen = false;
Assert(!prstate.all_frozen || prstate.all_visible);
/* Any error while applying the changes is critical */
@@ -841,29 +882,11 @@ heap_page_prune_and_freeze(PruneFreezeParams *params,
* on the standby with xids older than the youngest tuple this
* record will freeze will conflict.
*/
TransactionId frz_conflict_horizon = InvalidTransactionId;
TransactionId conflict_xid;
/*
* We can use the visibility_cutoff_xid as our cutoff for
* conflicts when the whole page is eligible to become all-frozen
* in the VM once we're done with it. Otherwise we generate a
* conservative cutoff by stepping back from OldestXmin.
*/
if (do_freeze)
{
if (prstate.all_frozen)
frz_conflict_horizon = prstate.visibility_cutoff_xid;
else
{
/* Avoids false conflicts when hot_standby_feedback in use */
frz_conflict_horizon = prstate.cutoffs->OldestXmin;
TransactionIdRetreat(frz_conflict_horizon);
}
}
if (TransactionIdFollows(frz_conflict_horizon, prstate.latest_xid_removed))
conflict_xid = frz_conflict_horizon;
if (TransactionIdFollows(prstate.frz_conflict_horizon,
prstate.latest_xid_removed))
conflict_xid = prstate.frz_conflict_horizon;
else
conflict_xid = prstate.latest_xid_removed;
@@ -887,30 +910,8 @@ heap_page_prune_and_freeze(PruneFreezeParams *params,
presult->nfrozen = prstate.nfrozen;
presult->live_tuples = prstate.live_tuples;
presult->recently_dead_tuples = prstate.recently_dead_tuples;
/*
* It was convenient to ignore LP_DEAD items in all_visible/all_frozen
* earlier on to make the choice of whether or not to freeze the page
* unaffected by the short-term presence of LP_DEAD items. These LP_DEAD
* items were effectively assumed to be LP_UNUSED items in the making. It
* doesn't matter which vacuum heap pass (initial pass or final pass) ends
* up setting the page all-frozen, as long as the ongoing VACUUM does it.
*
* Now that freezing has been finalized, unset all_visible and all_frozen
* if there are any LP_DEAD items on the page. It needs to reflect the
* present state of the page, as expected by our caller.
*/
if (prstate.all_visible && prstate.lpdead_items == 0)
{
presult->all_visible = prstate.all_visible;
presult->all_frozen = prstate.all_frozen;
}
else
{
presult->all_visible = false;
presult->all_frozen = false;
}
presult->all_visible = prstate.all_visible;
presult->all_frozen = prstate.all_frozen;
presult->hastup = prstate.hastup;
/*