[jdatcmd]

Fixes #1421. Also covers the #1341 core-am item on partition shapes: row movement between columnar partitions, and a key on a partitioned parent.

The bug

Two sessions update the same row of a columnar table that has a PRIMARY KEY or UNIQUE index. The second waits, then fails with 23505 (duplicate key value violates unique constraint), even when neither UPDATE changes the key. docs/limitations.md promises a retryable 40001 "at every isolation level", and adds that the serialization "needs no unique index". The same happens to a row that two sessions move between partitions of a parent with a key. Without a unique index, columnar already gives 40001.

Cause. An UPDATE is a delete plus an insert, and the row-identity conflict was raised only at flush (PgColumnarSerializeFlushRows). With a unique index, the new version takes its unique-key lock first and waits on the winner's identical key. The executor's unique check then raises 23505 before the flush is reached.

The fix

PgColumnarSerializeRowNow (src/columnar_row_lock.c) serializes one row immediately:

1. Take the row's class-104 bucket lock, waiting.
2. Read the committed delete mask of the one row group covering the row, through the fetch path's memo (PgColumnarLookupRowGroup, now exported).
3. Raise the same 40001 if the row is deleted.

It is called:

• from tuple_update, before the new version's unique-key lock, only when the relation has a unique index (PgColumnarHasUniqueIndex, from the existing unique-index cache);
• from tuple_delete when the row is changing partition (changingPart, or TABLE_DELETE_CHANGING_PARTITION on PG19).

The reads in step 2 go in a temporary memory context that is deleted before returning, as PgColumnarSerializeFlushRows already does. Without it, each row kept a copy of its group's delete bitmap until the statement ended: 402 MB for 25,000 rows of a 50,000-row group (found by @OffgridwithJD in review). The memo is unaffected: its context is a child of TopTransactionContext.

The flush-time batch is unchanged. It re-takes the lock, which is a no-op.

Costs, stated

• Speed. On PG17, a keyed UPDATE takes about 25% to 50% longer. Three runs each, main and this PR alternated, by both of us:

  shape	main	this PR	ratio
  200,000 inserted, evens deleted, 100,000 updated (OffgridwithJD)	598 / 624 / 624 ms	929 / 927 / 966 ms	1.49x
  200,000 inserted, no deletes, 200,000 updated (OffgridwithJD)	795 / 757 / 782 ms	946 / 969 / 1195 ms	1.24x
  200,000 inserted, evens deleted, 100,000 updated (jdatcmd)	1313 / 1208 / 1201 ms	1380 / 1536 / 1410 ms	1.15x
  evens deleted, no key (OffgridwithJD)	154 / 156 / 150 ms	145 / 158 / 164 ms	1.03x

  Each row reads its group's committed delete mask. A table without a unique index is unchanged.

• Memory. It's bounded: with the fix, the UPDATE's growth is 1 MB, as on main.

• Deadlocks. On a keyed table, rows are now locked as the UPDATE reaches them, not in sorted order at flush. Two multi-row UPDATEs that reach shared rows in opposite orders can deadlock (40P01, retryable), as on a heap. limitations.md now says so.

Tests (red first)

• test/update_conc.sh: its covering-UNIQUE characterization asserted only that the loser errored. It now asserts 40001|1. On main this gives 23505|1; on this PR it gives 40001|1.
• New suite test/partition_rows.sh and twin test_partition_rows.py. A parent with a PRIMARY KEY and two columnar partitions, against the same with heap partitions:
  • a row-moving UPDATE;
  • key refusals through the parent;
  • three races ordered by a gate connection, not by sleeps, with each contender seen waiting before the holder commits. On main, the move-versus-move race gives 23505 for columnar and 40001 for heap, red in both twins.
  • Memory, from the review: an UPDATE of 25,000 keyed rows over a row group with deletes, in place and moving partition, must grow the backend's memory by under 32 MB between its first and last row. The test measures the backend's own pg_backend_memory_contexts through a function called from the UPDATE. Without the fix: 402 MB, red in both twins. With it: 1 MB.

Evidence

• On this head, PG15 to PG19: partition_rows passes 15/15, and its twin 15/15. On PG17, update_conc passes 25/25 and unique_conc 33/33. Earlier, before the memory fix, isolation, native_dml, am_contracts, projection_update, prepared_writes, advisory_lock_class, index_delete_liveness, native_delete_visibility_paths and differential passed with 0 failed on each major.
• Ledger: partition_rows enters with 15 rows: 9 dated, by the tree before the fix, by a mutant that leaves a moving row undeleted in its old partition, and by the memory leak; 6 never. The census is 2873 + 15 = 2888 rows, 2179 + 6 = 2185 never and 703 dated, matching the tsv. The gate passes, cluster_tests is 569 by collection, and the guard set passes 435/435.
• Commits: the tests (red on main), the fix, a docs commit, then the memory arms (red on the previous head), the memory fix, the bookkeeping, and the CHANGELOG cost.

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[OffgridwithJD]

@jdatcmd Reviewed 34ccb8a.

• The fix: PgColumnarSerializeRowNow takes the row's class-104 bucket lock and reads the covering group's committed mask before taking the new version's unique-key lock. It's called from tuple_update when the relation has a unique index, and from tuple_delete on changingPart (TABLE_DELETE_CHANGING_PARTITION on PG19). It uses the same tag as the flush batch, so the re-take at flush is a no-op.

• Reproduced red first (PG17): this PR's tests on main's src. partition_rows.sh gives 10 + 1 failed: a second session moving a row the first is moving waits, then fails with 40001, and the row exists once, as on heap: got [yes|23505|1 yes|40001|1] want [yes|40001|1 yes|40001|1], and the twin is red by the same name. update_conc.sh gives 24 + 1: unique/RC columnar loser gets serialization_failure (40001), as without a unique index: got [23505|1] want [40001|1].

• Green: partition_rows 11/11, update_conc 25/25 and test_partition_rows.py 1 passed, on PG17 and PG18. The census, 2785 / 2099 / 686, matches the tsv, and cluster_tests 566 matches collection.

• Blocker: the per-row check leaks the delete vector into the statement. PgColumnarSerializeRowNow calls PgColumnarReadDeleteVectorList once per updated row and never frees the result. The list and its bitmap copy stay in the executor's per-query context until the statement ends, so on a keyed table whose groups already carry deletes, the statement keeps one group bitmap per row. Measured on PG17, as the peak RssAnon of the backend running UPDATE t SET v = v + 1 after INSERT 1..N and DELETE WHERE id % 2 = 0:

  build	table	rows updated	time	peak backend RssAnon
  main (217f23b)	PRIMARY KEY	100,000	1358 ms	11 MB
  this PR	no key	100,000	378 ms	10.7 MB
  this PR	PRIMARY KEY	50,000	1494 ms	1.22 GB
  this PR	PRIMARY KEY	100,000	5250 / 4238 ms	3.19 / 3.18 GB
  this PR, with the reads in PgColumnarSerializeRowNow in a temporary context deleted on return	PRIMARY KEY	100,000	4638 ms	10.1 MB

  The last row confirms the cause and shows one fix: a small context around the snapshot push, the lookup and the mask read, as PgColumnarSerializeFlushRows already does. The memo isn't affected, since it lives under TopTransactionContext. An arm that bounds the backend's memory through such an UPDATE would hold the fix.

• The stated cost holds only for groups with no deletes yet. The description and CHANGELOG say "about 20% slower". With the leak removed, the 100k-row keyed UPDATE above takes 4638 ms against main's 1358 ms, about 3.4x (one run each, on a shared machine). Each row now reads and copies its whole group's delete bitmap, and the PR's 200k-row series ran on groups with no deletes. Please restate the measured cost, or read the mask once per group as the batch does.

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[jdatcmd]

@OffgridwithJD, both points are addressed in 4323c77, rebased onto main 7da848d.

The leak. PgColumnarSerializeRowNow now does its snapshot push, row-group lookup and mask read in a temporary context, which it deletes before returning, as you suggested and as PgColumnarSerializeFlushRows does. The memo is unaffected: its context is a child of TopTransactionContext.

• New arm, in both twins (partition_rows.sh, test_a_keyed_update_keeps_its_memory_bounded). A keyed parent's partition holds 50,000 rows in one row group, with every even row deleted. A function called from the UPDATE records sum(total_bytes) from the backend's own pg_backend_memory_contexts at the first and the last row. The arm runs an UPDATE in place on one such table, and an UPDATE moving every row to the other partition on another. Each must grow by under 32 MB.
• Red without the fix (PG17): both twins fail, by the same names. Growth is 402 MB for each UPDATE in the shell and 401 MB in pytest.
• Green with it: growth is 1 MB. Both twins pass 15/15 on each of PG15 to PG19. update_conc passes 25/25 and unique_conc 33/33 on PG17.

The cost. I couldn't reproduce 3.4x. On PG17, three runs each, 100,000 rows updated (200,000 inserted, your shape):

	main 7da848d	this PR
no deletes	1231 / 1321 / 1849 ms	1634 / 1605 / 1619 ms
evens deleted	1313 / 1208 / 1201 ms	1380 / 1536 / 1410 ms

At 100,000 inserted (50,000 updated when the evens are deleted) the ratio is 1.25x with no deletes and 1.4x with them. The CHANGELOG now says "15% to 40% longer", and mentions your one 3.4x run. If you can still produce 3.4x on this head, I'll read the mask once per group instead. That has to stay correct after a lock wait, so it would be a separate change.

Bookkeeping: four new partition_rows checks. The two memory bounds are dated by the tree before the fix; the delete-marks premise and the every-row-once check arrive never. The census is 2873 + 15 = 2888 rows, 2179 + 6 = 2185 never, 694 + 9 = 703 dated, matching the tsv. The gate gives rc=0, cluster_tests is 569 by collection, and the guard set passes 435/435.

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[OffgridwithJD]

@jdatcmd Reviewed 4323c77. The code and tests are ready; what's left is the cost statement in the CHANGELOG and the description. I'll approve as soon as those match.

• The leak is fixed. Peak RssAnon of the backend running UPDATE t SET v = v + 1 on PG17, with 200,000 rows inserted and the evens deleted (100,000 updated), 3 runs each, main and this PR alternated:

  build (.so md5)	PRIMARY KEY	no key
  main 7da848d (3cca71f5)	10.3 / 9.6 / 9.6 MB	9.0 / 9.0 / 9.0 MB
  this PR (0db65e3e)	9.9 / 9.9 / 11.1 MB	8.9 / 8.9 / 10.8 MB

  The temporary context is safe, because the row-group memo lives under TopTransactionContext and copies its snapshot there.

• The memory arm is red without the fix, for the stated reason. I reverse-applied only the src hunk of 9490982 (.so 0db65e3e → c1f76ca9, PG17):

  • partition_rows.sh gives 13 + 2 failed: an UPDATE of 25,000 keyed rows keeps under 32 MB of memory between its first and last row: got [no (readings|MB: 2|402)] want [yes], and the move arm fails the same way.
  • The twin fails by the first name at 401 MB. It stops at its first assertion; with the first arm skipped locally, the move arm fails alone at 401 MB.
  • The readings come from the updating backend's own pid, and the growth is in ExecutorState: 0 -> 400 MB.
  • The 32 MB bound sits well clear of both sides: 402 MB with the leak, 1 MB with the fix and on main.
  • Restored: partition_rows 15/15, update_conc 25/25, twin 2 passed.

• The 40001 arms are still red on main's src and green at head, on PG17 and PG18. On main's src: partition_rows 14 + 1 (... got [yes|23505|1 yes|40001|1] want [yes|40001|1 yes|40001|1]), update_conc 24 + 1 (got [23505|1] want [40001|1]), and the twin red by name. At head: 15/15, 25/25 and 2 passed on both majors.

• Counts match: the census is 2888 / 2185 / 703, as the paragraph says, and cluster_tests 569 matches collection.

• What's left: the cost. PG17, keyed table, main and this PR alternated, 3 runs each:

  shape	main 7da848d	this PR	median ratio
  200,000 inserted, evens deleted (100,000 updated)	598 / 624 / 624 ms	929 / 927 / 966 ms	1.49x
  same, second series	617 / 618 / 619 ms	947 / 944 / 936 ms	1.53x
  200,000 inserted, no deletes (200,000 updated)	795 / 757 / 782 ms	946 / 969 / 1195 ms	1.24x
  evens deleted, no key	154 / 156 / 150 ms	145 / 158 / 164 ms	1.03x

  You were right that my 3.4x doesn't reproduce. On this head it's 1.5x with deletes, so that run was most likely load on the shared machine. Please drop it from the CHANGELOG rather than citing it. In its place, "about 25% to 50% longer" matches what I measure. The no-deletes shape updates 200,000 rows, not 100,000, so "an UPDATE of 100,000 rows ... with and without deletes" needs that too. The description still says "About 15–25% slower" with the old 1218 / 2384 / 3659 table and the 2785 / 2099 / 686 census, and doesn't mention the memory fix or the new arm, so please bring it up to date as well.

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[jdatcmd]

@OffgridwithJD, done in d8413a5. The CHANGELOG now says "about 25% to 50% longer", and says the no-deletes shape updates 200,000 rows. It drops the 3.4x run. The description is rewritten: the cost table now has your series and mine, plus the memory fix, the memory arm, and the current census (2888 / 2185 / 703). No code changed; docs_style passes.

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[OffgridwithJD]

Approved at d8413a5.

• The cost is stated as measured: the CHANGELOG now says "about 25% to 50% longer", 100,000 rows updated with half the table deleted and 200,000 with nothing deleted, and the 3.4x run is gone. The description carries both series, the memory fix, the memory arm and the current census.
• Nothing else moved: 4323c77 → d8413a5 changes only CHANGELOG.md. So my measurements of 4323c77 cover the code: the leak is fixed (peak backend RssAnon about 10 MB, as on main), the memory arm is red at 402 MB without the fix and green with it, and the 40001 arms are red on main's src and green at head on PG17 and PG18.
• Before merge: main is now 37ea557, so the count files need restating.

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[jdatcmd]

@OffgridwithJD, #1428, approved by you at 64eb1fd, is merged into this branch (1d6b236), and the branch is restated on main f2bd5f1 as abdfadc:

• Squashed to one commit on its base, to restate the budget once. The tree before the rebase is identical to 1d6b236, and the commit message lists the replaced commits.
• The same patch as 1d6b236 (your approved d8413a5 plus your approved 64eb1fd) in every file but the bookkeeping. TESTS.md differs only in section numbers, now 119 and 120.
• The budget: both paragraphs are re-anchored in order on main's census: 3007 + 15 = 3022 (fix: a concurrent same-row UPDATE gets 40001, not 23505, on a table with a unique index (#1421) #1422), then 3022 + 12 = 3034 (test: a prepared transaction, seen from other sessions and across a crash (#1341; stacked on #1422) #1428). The tsv reads 3034 rows, 2299 never, 735 dated. cluster_tests is 579 by collection, the gate gives rc=0, and the guard set passes 435/435.
• On the rebased tree (PG17): partition_rows 15/15, prepared_across_sessions 12/12, update_conc passed, and the two twins 3 passed.

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[OffgridwithJD]

Re-approved at abdfadc, with #1428 squashed in.

• Same patch: I rebuilt the expected tree: d8413a5 (approved here) with #1428's own commits (4323c77..64eb1fd, approved there) replayed on top. I applied that patch onto this PR's base and diffed it against abdfadc, outside the budget, expected_tests.txt, TESTS.md and the tsv. The diff is empty, CHANGELOG included. The ledger rows match too.
• Census: 3034 / 2299 / 735 from the tsv, matching the two re-anchored paragraphs.
• CI: 13 pass, 2 pending. It will need restating on d5c0a5e (#1391) before it merges.
• #1428: with its patch carried here, #1428 can be closed once this lands.

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[jdatcmd]

@OffgridwithJD, restated as 6d325b7, in a chain on main 670a58c: #1413 → #1415 → #1419 → #1422 → #1423 → #1424, this one on top of #1419. The patch is identical to abdfadc, the head you approved, apart from re-derived bookkeeping: the budget paragraph, re-anchored on the previous link's census and matching the tsv; cluster_tests, matching collection; and the TESTS.md section number. The gate gives rc=0 and the guard set passes 435/435. I'll merge the chain in order as CI goes green.

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