Thanks for the review. On Mon, Sep 21, 2026 at 4:37 AM Andrey Borodin <[email protected]> wrote: > > On Fri, Sep 18, 2026, Melanie Plageman wrote: > > Pruning and freezing > > always required a cleanup lock in recovery > > Could 0002 use just do_prune instead of do_prune || do_freeze? > The comment for XLHP_CLEANUP_LOCK allows freezing under an ordinary > exclusive lock. Before pruning and freezing shared a WAL record, > freeze-only replay in heap_xlog_freeze_page() used normal exclusive > lock IIUC. > > With 0002 I observe VACUUM FREEZE on a page containing only live > tuples to terminate a standby cursor holding a pin. Using just do_prune > lets replay finish without canceling the cursor.
Good point. I see this brought up also in [1] > 0001, 0003 and 0004 look good to me. I tried 0003/0004 with a manual > primary/standby test, but found nothing interesting. I've tightened up the commit messages in the latest version and changed 0002 as you suggested. I'll commit 0001-0003 after beta4 is tagged tomorrow. 0004 I'm going to think about just a bit longer (and would be master only). - Melanie [1] https://www.postgresql.org/message-id/CAHg%2BQDf3NcB3vOAqQ5EFcV5DYYUZ9snCSJAU-x1JDbtcjK3eBQ%40mail.gmail.com
From c722237f3a865f785d52c1e86803e3853a43a130 Mon Sep 17 00:00:00 2001 From: Melanie Plageman <[email protected]> Date: Fri, 18 Sep 2026 15:45:35 -0400 Subject: [PATCH v2 1/4] Assert correct VM page passed to pruning Before pruning a heap page, we get the current status of the corresponding VM page. If the passed in vmbuffer isn't the right one, visibilitymap_get_status() will silently unpin it and pin the correct page. Pruning assumes the caller manages the vmbuffer lifecycle, so this would leave the caller with a stale VM reference and would leak the new VM pin. To avoid mistakes in development, assert that the correct VM page is pinned before beginning. Reported-by: Melanie Plageman <[email protected]> Reviewed-by: Andrey Borodin <[email protected]> Discussion: https://postgr.es/m/CAAKRu_amj7qLF4c=9ijd=708Fu2G8gg-2EqwBu=acdahu2s...@mail.gmail.com --- src/backend/access/heap/pruneheap.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c index 50f810c8830..5778ba49a05 100644 --- a/src/backend/access/heap/pruneheap.c +++ b/src/backend/access/heap/pruneheap.c @@ -443,7 +443,13 @@ prune_freeze_setup(PruneFreezeParams *params, prstate->buffer = params->buffer; prstate->page = BufferGetPage(params->buffer); - Assert(BufferIsValid(params->vmbuffer)); + /* + * The caller must have pinned the VM page covering this heap block. If it + * hadn't, visibilitymap_get_status() below would silently release the + * caller's pin and take its own, leaving the caller holding a stale + * buffer and leaking ours. + */ + Assert(visibilitymap_pin_ok(prstate->block, params->vmbuffer)); prstate->vmbuffer = params->vmbuffer; prstate->new_vmbits = 0; prstate->old_vmbits = visibilitymap_get_status(prstate->relation, -- 2.43.0
From 8ea3610c68f78ba902a0ef0ac71226e300f5dc6e Mon Sep 17 00:00:00 2001 From: Melanie Plageman <[email protected]> Date: Mon, 21 Sep 2026 11:21:57 -0400 Subject: [PATCH v2 2/4] Avoid replay cleanup locks for freeze-only and VM-only records 6dbb490261a combined pruning and freezing in a single WAL record but unconditionally requested a cleanup lock during replay. Prior to version 17, freeze-only records were replayed under an ordinary exclusive lock. 1252a4ee286 subsequently folded visibility map updates into the same records, extending the unnecessary cleanup-lock requirement to VM-only updates starting in version 19. Request a cleanup lock in redo only when the record actually prunes. Freezing and visibility updates do not move or remove tuple storage, so they need not wait for other buffer pins to be released. This avoids unnecessary replay delays and cancellation of standby queries holding buffer pins. The fix is the same for all versions, though it addresses two oversights in master/19 and one in 18/17. Reported-by: Satyanarayana Narlapuram <[email protected]> Reported-by: Andrey Borodin <[email protected]> Reviewed-by: Andrey Borodin <[email protected]> Discussion: https://postgr.es/m/[email protected] Discussion: https://postgr.es/m/cahg+qdf3ncb3voaqq5efcv5dyyuz9sncsjau-x1jdbtcjk3...@mail.gmail.com Discussion: https://postgr.es/m/[email protected] --- src/backend/access/heap/pruneheap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c index 5778ba49a05..314b03b56f2 100644 --- a/src/backend/access/heap/pruneheap.c +++ b/src/backend/access/heap/pruneheap.c @@ -1337,7 +1337,7 @@ heap_page_prune_and_freeze(PruneFreezeParams *params, do_set_vm ? prstate.vmbuffer : InvalidBuffer, do_set_vm ? prstate.new_vmbits : 0, conflict_xid, - true, /* cleanup lock */ + do_prune, /* cleanup lock */ params->reason, prstate.frozen, prstate.nfrozen, prstate.redirected, prstate.nredirected, -- 2.43.0
From f45b9ccdc22c5d96b33a53c8d0101447d1774278 Mon Sep 17 00:00:00 2001 From: Melanie Plageman <[email protected]> Date: Fri, 18 Sep 2026 15:49:33 -0400 Subject: [PATCH v2 3/4] Correct on-access VM setting heuristic The heuristic to avoid setting the VM during on-access pruning when doing so would emit an extra FPI missed a few cases. First it missed temp and unlogged tables. Those will never emit an FPI, so they can always set the VM if the page is all-visible. It also missed that if hint bits are not WAL-logged, setting only the VM passes REGBUF_NO_IMAGE and forbids a heap FPI in the WAL record. The third is more subtle: If the page is all-visible, the new prune xid will be InvalidTransactionId. On-access pruning only executes when the current pd_prune_xid is valid and visible. So, when on-access pruning finds the page all-visible, it will always clear pd_prune_xid, modifying the page. Knowing this means we can set the VM without emitting an extra heap FPI in more cases. When hint bits are WAL-logged, if the heap buffer is clean, modifying pd_prune_xid will emit an FPI if one is required. There is no reason to try to avoid an FPI by not setting the VM. So, set the VM in this case. However, when the heap buffer is already dirty, modifying pd_prune_xid can avoid an FPI; so if the page hasn't been logged since the last checkpoint, setting it all-visible will emit an extra heap page FPI. We will still avoid setting the VM in this case. Expanding the cases where we set the VM on-access could be considered an enhancement. However, it is being backpatched because since 378a216187ae pd_prune_xid is set on insert, and if we execute a prune cycle and skip setting the VM because of an incorrect heuristic, we have added new wasted work in PG 19. This commit adds some tests covering these cases. It also updates one of the existing temp table tests to avoid exceeding the pin limit. Setting the VM and FSM on-access when querying temp tables takes more local pins and can run into the limit with fewer heap buffers pinned. Reported-by: Melanie Plageman <[email protected]> Author: Melanie Plageman <[email protected]> Reviewed-by: Andrey Borodin <[email protected]> Discussion: https://postgr.es/m/CAAKRu_amj7qLF4c=9ijd=708Fu2G8gg-2EqwBu=acdahu2s...@mail.gmail.com --- .../pg_visibility/expected/pg_visibility.out | 76 +++++++++++++++++++ contrib/pg_visibility/sql/pg_visibility.sql | 34 +++++++++ src/backend/access/heap/pruneheap.c | 37 +++++++-- src/test/regress/expected/temp.out | 2 +- src/test/regress/sql/temp.sql | 2 +- 5 files changed, 142 insertions(+), 9 deletions(-) diff --git a/contrib/pg_visibility/expected/pg_visibility.out b/contrib/pg_visibility/expected/pg_visibility.out index d26f0ab7589..a7aa8487cc3 100644 --- a/contrib/pg_visibility/expected/pg_visibility.out +++ b/contrib/pg_visibility/expected/pg_visibility.out @@ -248,6 +248,82 @@ select pg_visibility_map_summary('test_vac_unmodified_heap'); (1,1) (1 row) +-- Test that on-access pruning during a read-only scan sets the VM. Temp tables +-- are used because their visibility horizon depends only on this backend and no +-- other process can pin their buffers, so the conditional cleanup lock needed for +-- pruning is always available. +create temp table test_on_access_vm(a int, b text) with (fillfactor = 90); +insert into test_on_access_vm select g, repeat('x', 99) + from generate_series(1, 500) g; +-- HOT-update a few rows on every page. The new versions fit in the space +-- reserved by the fillfactor, and afterwards each page has too little free +-- space to escape on-access pruning. +update test_on_access_vm set b = b where a % 20 = 0; +select pg_visibility_map_summary('test_on_access_vm'); + pg_visibility_map_summary +--------------------------- + (0,0) +(1 row) + +-- A read-only scan that prunes tuples sets the VM +select count(*) from test_on_access_vm; + count +------- + 500 +(1 row) + +select pg_visibility_map_summary('test_on_access_vm'); + pg_visibility_map_summary +--------------------------- + (9,0) +(1 row) + +select * from pg_check_visible('test_on_access_vm'); + t_ctid +-------- +(0 rows) + +-- A read-only scan of newly inserted data sets the VM +create temp table test_on_access_vm_insert_only(a int, b text); +insert into test_on_access_vm_insert_only select g, repeat('x', 99) + from generate_series(1, 500) g; +select pg_visibility_map_summary('test_on_access_vm_insert_only'); + pg_visibility_map_summary +--------------------------- + (0,0) +(1 row) + +select count(*) from test_on_access_vm_insert_only; + count +------- + 500 +(1 row) + +select pg_visibility_map_summary('test_on_access_vm_insert_only'); + pg_visibility_map_summary +--------------------------- + (8,0) +(1 row) + +select * from pg_check_visible('test_on_access_vm_insert_only'); + t_ctid +-------- +(0 rows) + +create temp table test_on_access_vm_modify(a int, b text) with (fillfactor = 90); +insert into test_on_access_vm_modify select g, repeat('x', 99) + from generate_series(1, 500) g; +-- Create some dead rows for the next update's on-access pruning to clean up +update test_on_access_vm_modify set b = b where a % 20 = 0; +-- A scan by a query that modifies the relation prunes but does not set the VM. +-- This matches no rows, but scans every page as the query's result relation. +update test_on_access_vm_modify set b = b where a = -1; +select pg_visibility_map_summary('test_on_access_vm_modify'); + pg_visibility_map_summary +--------------------------- + (0,0) +(1 row) + -- test copy freeze create table copyfreeze (a int, b char(1500)); -- load all rows via COPY FREEZE and ensure that all pages are set all-visible diff --git a/contrib/pg_visibility/sql/pg_visibility.sql b/contrib/pg_visibility/sql/pg_visibility.sql index 0888adb96a6..f292679bd58 100644 --- a/contrib/pg_visibility/sql/pg_visibility.sql +++ b/contrib/pg_visibility/sql/pg_visibility.sql @@ -114,6 +114,40 @@ SELECT (flags & x'0004'::int) <> 0 vacuum test_vac_unmodified_heap; select pg_visibility_map_summary('test_vac_unmodified_heap'); +-- Test that on-access pruning during a read-only scan sets the VM. Temp tables +-- are used because their visibility horizon depends only on this backend and no +-- other process can pin their buffers, so the conditional cleanup lock needed for +-- pruning is always available. +create temp table test_on_access_vm(a int, b text) with (fillfactor = 90); +insert into test_on_access_vm select g, repeat('x', 99) + from generate_series(1, 500) g; +-- HOT-update a few rows on every page. The new versions fit in the space +-- reserved by the fillfactor, and afterwards each page has too little free +-- space to escape on-access pruning. +update test_on_access_vm set b = b where a % 20 = 0; +select pg_visibility_map_summary('test_on_access_vm'); +-- A read-only scan that prunes tuples sets the VM +select count(*) from test_on_access_vm; +select pg_visibility_map_summary('test_on_access_vm'); +select * from pg_check_visible('test_on_access_vm'); +-- A read-only scan of newly inserted data sets the VM +create temp table test_on_access_vm_insert_only(a int, b text); +insert into test_on_access_vm_insert_only select g, repeat('x', 99) + from generate_series(1, 500) g; +select pg_visibility_map_summary('test_on_access_vm_insert_only'); +select count(*) from test_on_access_vm_insert_only; +select pg_visibility_map_summary('test_on_access_vm_insert_only'); +select * from pg_check_visible('test_on_access_vm_insert_only'); +create temp table test_on_access_vm_modify(a int, b text) with (fillfactor = 90); +insert into test_on_access_vm_modify select g, repeat('x', 99) + from generate_series(1, 500) g; +-- Create some dead rows for the next update's on-access pruning to clean up +update test_on_access_vm_modify set b = b where a % 20 = 0; +-- A scan by a query that modifies the relation prunes but does not set the VM. +-- This matches no rows, but scans every page as the query's result relation. +update test_on_access_vm_modify set b = b where a = -1; +select pg_visibility_map_summary('test_on_access_vm_modify'); + -- test copy freeze create table copyfreeze (a int, b char(1500)); diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c index 314b03b56f2..0f1c1126765 100644 --- a/src/backend/access/heap/pruneheap.c +++ b/src/backend/access/heap/pruneheap.c @@ -991,16 +991,39 @@ heap_page_will_set_vm(PruneState *prstate, PruneReason reason, return false; /* - * If this is an on-access call and we're not actually pruning, avoid - * setting the visibility map if it would newly dirty the heap page or, if - * the page is already dirty, if doing so would require including a - * full-page image (FPI) of the heap page in the WAL. + * If this is an on-access call and we're not actually pruning or + * freezing, consider whether setting the VM would cost us an additional + * heap page FPI. If the relation isn't WAL-logged, or if hint bits are + * not WAL-logged, setting the VM won't include a heap page FPI (the + * latter passes REGBUF_NO_IMAGE for the heap page), apart from a page + * that has never been WAL-logged, which we don't bother about here. */ if (reason == PRUNE_ON_ACCESS && !do_prune && !do_freeze && - (!BufferIsDirty(prstate->buffer) || XLogCheckBufferNeedsBackup(prstate->buffer))) + RelationNeedsWAL(prstate->relation) && XLogHintBitIsNeeded()) { - prstate->set_all_visible = prstate->set_all_frozen = false; - return false; + /* + * Because the page is known to be all-visible, we will clear + * pd_prune_xid regardless of whether we actually set the page + * all-visible in the VM. That clear is a hint update which is not + * WAL-logged, other than an FPI for torn-page protection, so in some + * cases we want to avoid setting the VM if doing so would cost us a + * heap page FPI that clearing pd_prune_xid wouldn't have. + * + * Since hint bits are WAL-logged, if the buffer is clean, clearing + * pd_prune_xid will already emit a heap page FPI if one is needed, so + * there's no reason to avoid setting the VM. + * + * However, if the heap buffer is already dirty, clearing pd_prune_xid + * will never emit an FPI. So avoid setting the VM if the page hasn't + * been WAL-logged since the current checkpoint began, as the record + * setting the VM would then include a heap page FPI. + */ + if (BufferIsDirty(prstate->buffer) && + XLogCheckBufferNeedsBackup(prstate->buffer)) + { + prstate->set_all_visible = prstate->set_all_frozen = false; + return false; + } } prstate->new_vmbits = VISIBILITYMAP_ALL_VISIBLE; diff --git a/src/test/regress/expected/temp.out b/src/test/regress/expected/temp.out index a50c7ae88a9..ae96d4a0272 100644 --- a/src/test/regress/expected/temp.out +++ b/src/test/regress/expected/temp.out @@ -485,7 +485,7 @@ FETCH NEXT FROM c_3; (1 row) -- new cursors with pins can be created after subtrans rollback -SELECT test_temp_pin(10, 94); +SELECT test_temp_pin(10, 93); test_temp_pin --------------- diff --git a/src/test/regress/sql/temp.sql b/src/test/regress/sql/temp.sql index d50472ddced..708187c6e86 100644 --- a/src/test/regress/sql/temp.sql +++ b/src/test/regress/sql/temp.sql @@ -369,7 +369,7 @@ ROLLBACK TO SAVEPOINT rescue_me; FETCH NEXT FROM c_3; -- new cursors with pins can be created after subtrans rollback -SELECT test_temp_pin(10, 94); +SELECT test_temp_pin(10, 93); -- Check that read streams deal with lower number of pins available SELECT count(*), max(a) max_a, min(a) min_a, max(cnt) max_cnt FROM test_temp; -- 2.43.0
From 2e47b563601e6d1ba4b4fd80f60733f23342de72 Mon Sep 17 00:00:00 2001 From: Melanie Plageman <[email protected]> Date: Fri, 18 Sep 2026 15:45:03 -0400 Subject: [PATCH v2 4/4] Retain newest live xid as prune hint after visibility horizon rejection When a page contains committed live tuples that are not yet visible to all snapshots, pruning cannot mark it all-visible. These tuples do not contribute to new_prune_xid, so pruning can clear pd_prune_xid and prevent subsequent on-access scans from reconsidering the page after the horizon advances. Vacuum scans the page regardless of pd_prune_xid, so the only change to its behavior is that it may dirty the page to set pd_prune_xid when it otherwise wouldn't have. Record the newest live xmin as a retry hint when the visibility horizon prevents setting the VM. Preserve any earlier pruning opportunity already recorded so that dead tuples can still be reclaimed sooner. Reviewed-by: Andrey Borodin <[email protected]> Discussion: https://postgr.es/m/CAAKRu_amj7qLF4c=9ijd=708Fu2G8gg-2EqwBu=acdahu2s...@mail.gmail.com --- src/backend/access/heap/pruneheap.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c index 0f1c1126765..5113e73895b 100644 --- a/src/backend/access/heap/pruneheap.c +++ b/src/backend/access/heap/pruneheap.c @@ -1209,8 +1209,20 @@ heap_page_prune_and_freeze(PruneFreezeParams *params, GlobalVisTestXidConsideredRunning(prstate.vistest, prstate.newest_live_xid, true)) + { prstate.set_all_visible = prstate.set_all_frozen = false; + /* + * Preserve an opportunity to set the VM on-access once the newest + * live xmin is visible to everyone. Retain any earlier pruning + * opportunity already recorded, so that we can still reclaim dead + * tuples sooner. + */ + if (!TransactionIdIsValid(prstate.new_prune_xid) || + TransactionIdPrecedes(prstate.newest_live_xid, prstate.new_prune_xid)) + prstate.new_prune_xid = prstate.newest_live_xid; + } + /* * If checksums are enabled, calling heap_prune_satisfies_vacuum() while * checking tuple visibility information in prune_freeze_plan() may have -- 2.43.0
