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Remove retry loop in heap_page_prune().
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The retry loop is needed because heap_page_prune() calls
HeapTupleSatisfiesVacuum() and then lazy_scan_prune() does the same
thing again, and they might get different answers due to concurrent
clog updates.  But this patch makes heap_page_prune() return the
HeapTupleSatisfiesVacuum() results that it computed back to the
caller, which allows lazy_scan_prune() to avoid needing to recompute
those values in the first place. That's nice both because it eliminates
the need for a retry loop and also because it's cheaper.

Melanie Plageman, reviewed by David Geier, Andres Freund, and me.

Discussion: https://postgr.es/m/CAAKRu_br124qsGJieuYA0nGjywEukhK1dKBfRdby_4yY3E9SXA%40mail.gmail.com
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@robertmhaas
robertmhaas committed Oct 2, 2023
1 parent e64c733 commit 1ccc1e0
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Showing 3 changed files with 55 additions and 49 deletions.
36 changes: 17 additions & 19 deletions src/backend/access/heap/pruneheap.c
View file
Expand Up @@ -53,16 +53,6 @@ typedef struct
* 1. Otherwise every access would need to subtract 1.
*/
bool marked[MaxHeapTuplesPerPage + 1];

/*
* Tuple visibility is only computed once for each tuple, for correctness
* and efficiency reasons; see comment in heap_page_prune() for details.
* This is of type int8[], instead of HTSV_Result[], so we can use -1 to
* indicate no visibility has been computed, e.g. for LP_DEAD items.
*
* Same indexing as ->marked.
*/
int8 htsv[MaxHeapTuplesPerPage + 1];
} PruneState;

/* Local functions */
Expand All @@ -71,6 +61,7 @@ static HTSV_Result heap_prune_satisfies_vacuum(PruneState *prstate,
Buffer buffer);
static int heap_prune_chain(Buffer buffer,
OffsetNumber rootoffnum,
int8 *htsv,
PruneState *prstate);
static void heap_prune_record_prunable(PruneState *prstate, TransactionId xid);
static void heap_prune_record_redirect(PruneState *prstate,
Expand Down Expand Up @@ -240,6 +231,10 @@ heap_page_prune(Relation relation, Buffer buffer,
prstate.nredirected = prstate.ndead = prstate.nunused = 0;
memset(prstate.marked, 0, sizeof(prstate.marked));

/*
* presult->htsv is not initialized here because all ntuple spots in the
* array will be set either to a valid HTSV_Result value or -1.
*/
presult->ndeleted = 0;
presult->nnewlpdead = 0;

Expand Down Expand Up @@ -276,7 +271,7 @@ heap_page_prune(Relation relation, Buffer buffer,
/* Nothing to do if slot doesn't contain a tuple */
if (!ItemIdIsNormal(itemid))
{
prstate.htsv[offnum] = -1;
presult->htsv[offnum] = -1;
continue;
}

Expand All @@ -292,8 +287,8 @@ heap_page_prune(Relation relation, Buffer buffer,
if (off_loc)
*off_loc = offnum;

prstate.htsv[offnum] = heap_prune_satisfies_vacuum(&prstate, &tup,
buffer);
presult->htsv[offnum] = heap_prune_satisfies_vacuum(&prstate, &tup,
buffer);
}

/* Scan the page */
Expand All @@ -317,7 +312,8 @@ heap_page_prune(Relation relation, Buffer buffer,
continue;

/* Process this item or chain of items */
presult->ndeleted += heap_prune_chain(buffer, offnum, &prstate);
presult->ndeleted += heap_prune_chain(buffer, offnum,
presult->htsv, &prstate);
}

/* Clear the offset information once we have processed the given page. */
Expand Down Expand Up @@ -446,6 +442,8 @@ heap_prune_satisfies_vacuum(PruneState *prstate, HeapTuple tup, Buffer buffer)
/*
* Prune specified line pointer or a HOT chain originating at line pointer.
*
* Tuple visibility information is provided in htsv.
*
* If the item is an index-referenced tuple (i.e. not a heap-only tuple),
* the HOT chain is pruned by removing all DEAD tuples at the start of the HOT
* chain. We also prune any RECENTLY_DEAD tuples preceding a DEAD tuple.
Expand Down Expand Up @@ -473,7 +471,8 @@ heap_prune_satisfies_vacuum(PruneState *prstate, HeapTuple tup, Buffer buffer)
* Returns the number of tuples (to be) deleted from the page.
*/
static int
heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate)
heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum,
int8 *htsv, PruneState *prstate)
{
int ndeleted = 0;
Page dp = (Page) BufferGetPage(buffer);
Expand All @@ -494,7 +493,7 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate)
*/
if (ItemIdIsNormal(rootlp))
{
Assert(prstate->htsv[rootoffnum] != -1);
Assert(htsv[rootoffnum] != -1);
htup = (HeapTupleHeader) PageGetItem(dp, rootlp);

if (HeapTupleHeaderIsHeapOnly(htup))
Expand All @@ -517,7 +516,7 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate)
* either here or while following a chain below. Whichever path
* gets there first will mark the tuple unused.
*/
if (prstate->htsv[rootoffnum] == HEAPTUPLE_DEAD &&
if (htsv[rootoffnum] == HEAPTUPLE_DEAD &&
!HeapTupleHeaderIsHotUpdated(htup))
{
heap_prune_record_unused(prstate, rootoffnum);
Expand Down Expand Up @@ -585,7 +584,6 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate)
break;

Assert(ItemIdIsNormal(lp));
Assert(prstate->htsv[offnum] != -1);
htup = (HeapTupleHeader) PageGetItem(dp, lp);

/*
Expand All @@ -605,7 +603,7 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate)
*/
tupdead = recent_dead = false;

switch ((HTSV_Result) prstate->htsv[offnum])
switch (htsv_get_valid_status(htsv[offnum]))
{
case HEAPTUPLE_DEAD:
tupdead = true;
Expand Down
43 changes: 13 additions & 30 deletions src/backend/access/heap/vacuumlazy.c
View file
Expand Up @@ -1524,12 +1524,13 @@ lazy_scan_new_or_empty(LVRelState *vacrel, Buffer buf, BlockNumber blkno,
* of complexity just so we could deal with tuples that were DEAD to VACUUM,
* but nevertheless were left with storage after pruning.
*
* The approach we take now is to restart pruning when the race condition is
* detected. This allows heap_page_prune() to prune the tuples inserted by
* the now-aborted transaction. This is a little crude, but it guarantees
* that any items that make it into the dead_items array are simple LP_DEAD
* line pointers, and that every remaining item with tuple storage is
* considered as a candidate for freezing.
* As of Postgres 17, we circumvent this problem altogether by reusing the
* result of heap_page_prune()'s visibility check. Without the second call to
* HeapTupleSatisfiesVacuum(), there is no new HTSV_Result and there can be no
* disagreement. We'll just handle such tuples as if they had become fully dead
* right after this operation completes instead of in the middle of it. Note that
* any tuple that becomes dead after the call to heap_page_prune() can't need to
* be frozen, because it was visible to another session when vacuum started.
*/
static void
lazy_scan_prune(LVRelState *vacrel,
Expand All @@ -1542,8 +1543,6 @@ lazy_scan_prune(LVRelState *vacrel,
OffsetNumber offnum,
maxoff;
ItemId itemid;
HeapTupleData tuple;
HTSV_Result res;
PruneResult presult;
int tuples_frozen,
lpdead_items,
Expand All @@ -1563,8 +1562,6 @@ lazy_scan_prune(LVRelState *vacrel,
*/
maxoff = PageGetMaxOffsetNumber(page);

retry:

/* Initialize (or reset) page-level state */
pagefrz.freeze_required = false;
pagefrz.FreezePageRelfrozenXid = vacrel->NewRelfrozenXid;
Expand Down Expand Up @@ -1600,6 +1597,7 @@ lazy_scan_prune(LVRelState *vacrel,
offnum <= maxoff;
offnum = OffsetNumberNext(offnum))
{
HeapTupleHeader htup;
bool totally_frozen;

/*
Expand Down Expand Up @@ -1642,22 +1640,7 @@ lazy_scan_prune(LVRelState *vacrel,

Assert(ItemIdIsNormal(itemid));

ItemPointerSet(&(tuple.t_self), blkno, offnum);
tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
tuple.t_len = ItemIdGetLength(itemid);
tuple.t_tableOid = RelationGetRelid(rel);

/*
* DEAD tuples are almost always pruned into LP_DEAD line pointers by
* heap_page_prune(), but it's possible that the tuple state changed
* since heap_page_prune() looked. Handle that here by restarting.
* (See comments at the top of function for a full explanation.)
*/
res = HeapTupleSatisfiesVacuum(&tuple, vacrel->cutoffs.OldestXmin,
buf);

if (unlikely(res == HEAPTUPLE_DEAD))
goto retry;
htup = (HeapTupleHeader) PageGetItem(page, itemid);

/*
* The criteria for counting a tuple as live in this block need to
Expand All @@ -1678,7 +1661,7 @@ lazy_scan_prune(LVRelState *vacrel,
* (Cases where we bypass index vacuuming will violate this optimistic
* assumption, but the overall impact of that should be negligible.)
*/
switch (res)
switch (htsv_get_valid_status(presult.htsv[offnum]))
{
case HEAPTUPLE_LIVE:

Expand All @@ -1700,7 +1683,7 @@ lazy_scan_prune(LVRelState *vacrel,
{
TransactionId xmin;

if (!HeapTupleHeaderXminCommitted(tuple.t_data))
if (!HeapTupleHeaderXminCommitted(htup))
{
prunestate->all_visible = false;
break;
Expand All @@ -1710,7 +1693,7 @@ lazy_scan_prune(LVRelState *vacrel,
* The inserter definitely committed. But is it old enough
* that everyone sees it as committed?
*/
xmin = HeapTupleHeaderGetXmin(tuple.t_data);
xmin = HeapTupleHeaderGetXmin(htup);
if (!TransactionIdPrecedes(xmin,
vacrel->cutoffs.OldestXmin))
{
Expand Down Expand Up @@ -1764,7 +1747,7 @@ lazy_scan_prune(LVRelState *vacrel,
prunestate->hastup = true; /* page makes rel truncation unsafe */

/* Tuple with storage -- consider need to freeze */
if (heap_prepare_freeze_tuple(tuple.t_data, &vacrel->cutoffs, &pagefrz,
if (heap_prepare_freeze_tuple(htup, &vacrel->cutoffs, &pagefrz,
&frozen[tuples_frozen], &totally_frozen))
{
/* Save prepared freeze plan for later */
Expand Down
25 changes: 25 additions & 0 deletions src/include/access/heapam.h
View file
Expand Up @@ -198,8 +198,33 @@ typedef struct PruneResult
{
int ndeleted; /* Number of tuples deleted from the page */
int nnewlpdead; /* Number of newly LP_DEAD items */

/*
* Tuple visibility is only computed once for each tuple, for correctness
* and efficiency reasons; see comment in heap_page_prune() for details.
* This is of type int8[], instead of HTSV_Result[], so we can use -1 to
* indicate no visibility has been computed, e.g. for LP_DEAD items.
*
* This needs to be MaxHeapTuplesPerPage + 1 long as FirstOffsetNumber is
* 1. Otherwise every access would need to subtract 1.
*/
int8 htsv[MaxHeapTuplesPerPage + 1];
} PruneResult;

/*
* Pruning calculates tuple visibility once and saves the results in an array
* of int8. See PruneResult.htsv for details. This helper function is meant to
* guard against examining visibility status array members which have not yet
* been computed.
*/
static inline HTSV_Result
htsv_get_valid_status(int status)
{
Assert(status >= HEAPTUPLE_DEAD &&
status <= HEAPTUPLE_DELETE_IN_PROGRESS);
return (HTSV_Result) status;
}

/* ----------------
* function prototypes for heap access method
*
Expand Down

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