On Thu, Sep 21, 2023 at 3:53 PM Robert Haas <robertmh...@gmail.com> wrote:
>
> On Wed, Sep 13, 2023 at 1:29 PM Andres Freund <and...@anarazel.de> wrote:
>
> > > /*
> > > * The criteria for counting a tuple as live in this block
> > > need to
> > > @@ -1682,7 +1664,7 @@ retry:
> > > * (Cases where we bypass index vacuuming will violate this
> > > optimistic
> > > * assumption, but the overall impact of that should be
> > > negligible.)
> > > */
> > > - switch (res)
> > > + switch ((HTSV_Result) presult.htsv[offnum])
> > > {
> > > case HEAPTUPLE_LIVE:
> >
> > I think we should assert that we have a valid HTSV_Result here, i.e. not
> > -1. You could wrap the cast and Assert into an inline funciton as well.
>
> This isn't a bad idea, although I don't find it completely necessary either.
Attached v5 does this. Even though a value of -1 would hit the default
switch case and error out, I can see the argument for this validation
-- as all other places switching on an HTSV_Result are doing so on a
value which was always an HTSV_Result.
Once I started writing the function comment, however, I felt a bit
awkward. In order to make the function available to both pruneheap.c
and vacuumlazy.c, I had to put it in a header file. Writing a
function, available to anyone including heapam.h, which takes an int
and returns an HTSV_Result feels a bit odd. Do we want it to be common
practice to use an int value outside the valid enum range to store
"status not yet computed" for HTSV_Results?
Anyway, on a tactical note, I added the inline function to heapam.h
below the PruneResult definition since it is fairly tightly coupled to
the htsv array in PruneResult. All of the function prototypes are
under a comment that says "function prototypes for heap access method"
-- which didn't feel like an accurate description of this function. I
wonder if it makes sense to have pruneheap.c include vacuum.h and move
pruning specific stuff like this helper and PruneResult over there? I
can't remember why I didn't do this before, but maybe there is a
reason not to? I also wasn't sure if I needed to forward declare the
inline function or not.
Oh, and, one more note. I've dropped the former patch 0001 which
changed the function comment about off_loc above heap_page_prune(). I
have plans to write a separate patch adding an error context callback
for HOT pruning with the offset number and would include such a change
in that patch.
- Melanie
From aa1ec4fa1a689f9c6328d02fd13af1a42ecb0396 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplage...@gmail.com>
Date: Wed, 6 Sep 2023 16:54:41 -0400
Subject: [PATCH v5 2/2] Reuse heap_page_prune() tuple visibility statuses
heap_page_prune() obtains the HTSV_Result (tuple visibility status)
returned from HeapTupleSatisfiesVacuum() for every tuple on the page and
stores them in an array. By making this array available to
heap_page_prune()'s caller lazy_scan_prune(), we can avoid an additional
call to HeapTupleSatisfiesVacuum() when freezing the tuples and
recording LP_DEAD items for vacuum. This saves resources and eliminates
the possibility that vacuuming corrupted data results in a hang due to
endless retry looping.
This replaces the retry mechanism introduced in 8523492d4 to handle cases in
which a tuple's inserting transaction aborted between the visibility check in
heap_page_prune() and lazy_scan_prune()'s call to HeapTupleSatisfiesVacuum() --
rendering it dead but without a dead line pointer. We can instead reuse the
tuple's original visibility status, circumventing any disagreements.
Discussion: https://postgr.es/m/CAAKRu_br124qsGJieuYA0nGjywEukhK1dKBfRdby_4yY3E9SXA%40mail.gmail.com
---
src/backend/access/heap/pruneheap.c | 36 +++++++++++-------------
src/backend/access/heap/vacuumlazy.c | 42 ++++++++--------------------
src/include/access/heapam.h | 25 +++++++++++++++++
3 files changed, 54 insertions(+), 49 deletions(-)
diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index d5892a2db4..c5f1abd95a 100644
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -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 */
@@ -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,
@@ -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;
@@ -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;
}
@@ -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 */
@@ -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. */
@@ -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.
@@ -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);
@@ -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))
@@ -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);
@@ -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);
/*
@@ -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;
diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index fa77ef7f4a..b8268a5cef 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -1524,12 +1524,12 @@ 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. The tuple's TID won't be added to the array of dead tuple TIDs
+ * for vacuum, thus vacuum will never be tasked with reaping a tuple with
+ * storage.
*/
static void
lazy_scan_prune(LVRelState *vacrel,
@@ -1542,8 +1542,6 @@ lazy_scan_prune(LVRelState *vacrel,
OffsetNumber offnum,
maxoff;
ItemId itemid;
- HeapTupleData tuple;
- HTSV_Result res;
PruneResult presult;
int tuples_frozen,
lpdead_items,
@@ -1563,8 +1561,6 @@ lazy_scan_prune(LVRelState *vacrel,
*/
maxoff = PageGetMaxOffsetNumber(page);
-retry:
-
/* Initialize (or reset) page-level state */
pagefrz.freeze_required = false;
pagefrz.FreezePageRelfrozenXid = vacrel->NewRelfrozenXid;
@@ -1600,6 +1596,7 @@ retry:
offnum <= maxoff;
offnum = OffsetNumberNext(offnum))
{
+ HeapTupleHeader htup;
bool totally_frozen;
/*
@@ -1642,22 +1639,7 @@ retry:
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
@@ -1678,7 +1660,7 @@ retry:
* (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:
@@ -1700,7 +1682,7 @@ retry:
{
TransactionId xmin;
- if (!HeapTupleHeaderXminCommitted(tuple.t_data))
+ if (!HeapTupleHeaderXminCommitted(htup))
{
prunestate->all_visible = false;
break;
@@ -1710,7 +1692,7 @@ retry:
* 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))
{
@@ -1764,7 +1746,7 @@ retry:
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 */
diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
index 2d3f149e4f..62fac1d5d2 100644
--- a/src/include/access/heapam.h
+++ b/src/include/access/heapam.h
@@ -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
*
--
2.37.2
From 430ff3458f0ac523f1c623d9bc2669981cc0e536 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplage...@gmail.com>
Date: Wed, 6 Sep 2023 14:57:20 -0400
Subject: [PATCH v5 1/2] Move heap_page_prune output parameters into struct
Add PruneResult, a structure containing the output parameters and result
of heap_page_prune(). Reorganizing the results of heap_page_prune() into
a struct simplifies the function signature and provides a location for
future commits to store additional output parameters.
Discussion: https://postgr.es/m/CAAKRu_br124qsGJieuYA0nGjywEukhK1dKBfRdby_4yY3E9SXA%40mail.gmail.com
---
src/backend/access/heap/pruneheap.c | 33 +++++++++++++---------------
src/backend/access/heap/vacuumlazy.c | 17 +++++---------
src/include/access/heapam.h | 13 +++++++++--
src/tools/pgindent/typedefs.list | 1 +
4 files changed, 33 insertions(+), 31 deletions(-)
diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index 18193efa23..d5892a2db4 100644
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -155,15 +155,13 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
*/
if (PageIsFull(page) || PageGetHeapFreeSpace(page) < minfree)
{
- int ndeleted,
- nnewlpdead;
+ PruneResult presult;
- ndeleted = heap_page_prune(relation, buffer, vistest,
- &nnewlpdead, NULL);
+ heap_page_prune(relation, buffer, vistest, &presult, NULL);
/*
* Report the number of tuples reclaimed to pgstats. This is
- * ndeleted minus the number of newly-LP_DEAD-set items.
+ * presult.ndeleted minus the number of newly-LP_DEAD-set items.
*
* We derive the number of dead tuples like this to avoid totally
* forgetting about items that were set to LP_DEAD, since they
@@ -175,9 +173,9 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
* tracks ndeleted, since it will set the same LP_DEAD items to
* LP_UNUSED separately.
*/
- if (ndeleted > nnewlpdead)
+ if (presult.ndeleted > presult.nnewlpdead)
pgstat_update_heap_dead_tuples(relation,
- ndeleted - nnewlpdead);
+ presult.ndeleted - presult.nnewlpdead);
}
/* And release buffer lock */
@@ -204,21 +202,19 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
* (see heap_prune_satisfies_vacuum and
* HeapTupleSatisfiesVacuum).
*
- * Sets *nnewlpdead for caller, indicating the number of items that were
- * newly set LP_DEAD during prune operation.
- *
* off_loc is the offset location required by the caller to use in error
* callback.
*
- * Returns the number of tuples deleted from the page during this call.
+ * presult contains output parameters needed by callers such as the number of
+ * tuples removed and the number of line pointers newly marked LP_DEAD.
+ * heap_page_prune() is responsible for initializing it.
*/
-int
+void
heap_page_prune(Relation relation, Buffer buffer,
GlobalVisState *vistest,
- int *nnewlpdead,
+ PruneResult *presult,
OffsetNumber *off_loc)
{
- int ndeleted = 0;
Page page = BufferGetPage(buffer);
BlockNumber blockno = BufferGetBlockNumber(buffer);
OffsetNumber offnum,
@@ -244,6 +240,9 @@ heap_page_prune(Relation relation, Buffer buffer,
prstate.nredirected = prstate.ndead = prstate.nunused = 0;
memset(prstate.marked, 0, sizeof(prstate.marked));
+ presult->ndeleted = 0;
+ presult->nnewlpdead = 0;
+
maxoff = PageGetMaxOffsetNumber(page);
tup.t_tableOid = RelationGetRelid(prstate.rel);
@@ -318,7 +317,7 @@ heap_page_prune(Relation relation, Buffer buffer,
continue;
/* Process this item or chain of items */
- ndeleted += heap_prune_chain(buffer, offnum, &prstate);
+ presult->ndeleted += heap_prune_chain(buffer, offnum, &prstate);
}
/* Clear the offset information once we have processed the given page. */
@@ -419,9 +418,7 @@ heap_page_prune(Relation relation, Buffer buffer,
END_CRIT_SECTION();
/* Record number of newly-set-LP_DEAD items for caller */
- *nnewlpdead = prstate.ndead;
-
- return ndeleted;
+ presult->nnewlpdead = prstate.ndead;
}
diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index b3c24d68f0..fa77ef7f4a 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -1544,12 +1544,11 @@ lazy_scan_prune(LVRelState *vacrel,
ItemId itemid;
HeapTupleData tuple;
HTSV_Result res;
- int tuples_deleted,
- tuples_frozen,
+ PruneResult presult;
+ int tuples_frozen,
lpdead_items,
live_tuples,
recently_dead_tuples;
- int nnewlpdead;
HeapPageFreeze pagefrz;
int64 fpi_before = pgWalUsage.wal_fpi;
OffsetNumber deadoffsets[MaxHeapTuplesPerPage];
@@ -1572,7 +1571,6 @@ retry:
pagefrz.FreezePageRelminMxid = vacrel->NewRelminMxid;
pagefrz.NoFreezePageRelfrozenXid = vacrel->NewRelfrozenXid;
pagefrz.NoFreezePageRelminMxid = vacrel->NewRelminMxid;
- tuples_deleted = 0;
tuples_frozen = 0;
lpdead_items = 0;
live_tuples = 0;
@@ -1581,15 +1579,12 @@ retry:
/*
* Prune all HOT-update chains in this page.
*
- * We count tuples removed by the pruning step as tuples_deleted. Its
- * final value can be thought of as the number of tuples that have been
- * deleted from the table. It should not be confused with lpdead_items;
+ * We count the number of tuples removed from the page by the pruning step
+ * in presult.ndeleted. It should not be confused with lpdead_items;
* lpdead_items's final value can be thought of as the number of tuples
* that were deleted from indexes.
*/
- tuples_deleted = heap_page_prune(rel, buf, vacrel->vistest,
- &nnewlpdead,
- &vacrel->offnum);
+ heap_page_prune(rel, buf, vacrel->vistest, &presult, &vacrel->offnum);
/*
* Now scan the page to collect LP_DEAD items and check for tuples
@@ -1929,7 +1924,7 @@ retry:
}
/* Finally, add page-local counts to whole-VACUUM counts */
- vacrel->tuples_deleted += tuples_deleted;
+ vacrel->tuples_deleted += presult.ndeleted;
vacrel->tuples_frozen += tuples_frozen;
vacrel->lpdead_items += lpdead_items;
vacrel->live_tuples += live_tuples;
diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
index 6598c4d7d8..2d3f149e4f 100644
--- a/src/include/access/heapam.h
+++ b/src/include/access/heapam.h
@@ -191,6 +191,15 @@ typedef struct HeapPageFreeze
} HeapPageFreeze;
+/*
+ * Per-page state returned from pruning
+ */
+typedef struct PruneResult
+{
+ int ndeleted; /* Number of tuples deleted from the page */
+ int nnewlpdead; /* Number of newly LP_DEAD items */
+} PruneResult;
+
/* ----------------
* function prototypes for heap access method
*
@@ -284,9 +293,9 @@ extern TransactionId heap_index_delete_tuples(Relation rel,
/* in heap/pruneheap.c */
struct GlobalVisState;
extern void heap_page_prune_opt(Relation relation, Buffer buffer);
-extern int heap_page_prune(Relation relation, Buffer buffer,
+extern void heap_page_prune(Relation relation, Buffer buffer,
struct GlobalVisState *vistest,
- int *nnewlpdead,
+ PruneResult *presult,
OffsetNumber *off_loc);
extern void heap_page_prune_execute(Buffer buffer,
OffsetNumber *redirected, int nredirected,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index b5bbdd1608..8de90c4958 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2151,6 +2151,7 @@ ProjectionPath
PromptInterruptContext
ProtocolVersion
PrsStorage
+PruneResult
PruneState
PruneStepResult
PsqlScanCallbacks
--
2.37.2
