[OffgridwithJD]

For #1341, the unported suite native_groupagg_wide_cost (no twin, no ledger rows).

What changes

The suite now reaches the fallback it is named for. It guards #12. When no serial scan path survives add_path, the grouped vector aggregate (PgColumnarTryGroupAggPath) prices its own input scan, and must price it as the columnar scan node does (pgcolumnar_refined_scan_cost), not with the bare seqscan formula. With 40,000 rows a serial scan survives, so the fold was priced from it and the fallback never ran. I traced this with a NOTICE in each branch: SERIAL 700.18 under the suite's settings. The old suite is 3/3 with the fallback reverted to the bare formula.

Free parallelism (parallel_setup_cost, parallel_tuple_cost and min_parallel_table_scan_size at 0) lets a Gather displace the serial scan; the trace then reads FALLBACK refined=700.18 bare=403.00. With enable_hashagg and enable_sort off, the parallel finalize can't win, so the fold node is the plan and its cost can be read. Two new checks:

• "premise: with parallelism free, a Gather displaces the serial scan";
• "the fallback prices the fold as a surviving serial scan does": the fold node's cost through the fallback must equal its cost when a serial scan survives. On PG17 that is 800.19 both ways, and 503.01 with the bare formula.

A failed query can no longer pass. Each reading was SET ...; SET ...; SELECT ... with tail -1. A failed SELECT left the last SET tag on both sides, so they agreed: the old suite is 3/3 with its query naming a missing column. The settings now ride in PGOPTIONS, and check_text refuses an empty side (the new suite goes red on the same broken query).

The plan check names the fold. It counted lines matching Batch Fold|Custom Scan, and the reference plan's input is a Custom Scan (PgColumnarScan) too. It now reads Columnar Batch Fold: yes.

The twin (test_native_groupagg_wide_cost.py) covers all 5 checks under the shell's names, one check per test. A module fixture takes each reading in the shell's order, on a connection of its own with the settings in its options. A failed query is a query_error(). compare_to_bash: 5 literal matches, 0 missing. The suite moves from unported_suites.txt to COMPLETE.

Evidence

Each mutant was built clean (columnar_vector.c). Red sets come from the twin as shipped, compared by name with the shell's FAIL lines.

mutant	PG17 shell / twin	PG18 shell / twin
none	5/5 / 5 passed	same
the fallback prices its input with the bare seqscan formula (serialScanCost = seq_page_cost * pages + cpu_tuple_cost * ntuples)	1 red, identical: the fallback arm (got [503.01] want [800.19])	same 1
the grouped path is never planned (if (true || …) return;)	2 red, identical: the fold-path arm and the fallback arm (no fold cost on either side)	same 2
the batch fold drops the first row of each row group	1 red, identical: the result arm	same 1

Clean on PG15, PG16 and PG19: 5/5 and 5 passed.

The old suite, on PG17: 3/3 under the bare-formula mutant, and 3/3 with the query naming a missing column.

Ledger

5 rows: 3 dated by the three mutations, 2 never (the Gather premise and "backend alive").

Bookkeeping, over main 1c7a97a3 (after #1531):

• suites_not_covered 183 -> 182.
• Census 3085 -> 3087 never, 1402 -> 1405 dated, 4487 -> 4492 rows.
• Gate against origin/main: rc=0. Orphan scan: 0. Sorted.
• cluster_tests 990 -> 995 by collection; guards 442 passed.
• TESTS.md §183, after test: native_agg_addcolumn gains its pytest twin, and its oracle can fail (#1341) #1530's §182.

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[jdatcmd]

Approving the substance at cb5eee51. The PR is DIRTY over #1528 (31cfc7b0), so it needs a restate. I'll re-check the restated head (tree identity outside the bookkeeping, census, collection, guards) and approve that head before merging.

Reproduced on PG17. I wrote the mutants myself from columnar_vector.c, each built clean. Red sets were compared by name, counting the shell's "a side is empty" form and the twin's VacuityError forms:

mutant	shell	twin	same?
none	5/5	5 passed	—
fallback prices its input with the bare formula (seq_page_cost * pages + cpu_tuple_cost * ntuples)	1: the fallback arm	1	yes
the grouped path is never planned (if (true || reloptkind != RELOPT_BASEREL) return;)	2: fold-path arm, fallback arm (empty side)	2	yes
mine: the loop that picks a surviving serial path no longer skips parallel and Gather paths	1: the fallback arm	1	yes

My mutant was the one I most wanted answered: does the fallback arm see a Gather posing as the serial survivor? It does. The fallback never runs, the fold borrows the Gather's cost, and the arm goes red. So the arm pins the fallback itself and not only the formula inside it.

I did not rerun the drop-first-row mutant. The result arm compares the fold with a non-fold reference, and check_text refuses an empty side, so it can't pass vacuously.

Also checked:

• Ledger: 5 rows. The 3 dated rows each name the mutation that reddened them. The 2 never rows are the Gather premise and "backend alive". The majors are 15–19, which matches your clean runs on all five.
• The old suite's defects: the tail -1 reading of a SET tag, and the plan check matching the reference's own Custom Scan. Both were real, and both are fixed the right way: PGOPTIONS, plus the fold's own Columnar Batch Fold: yes line.

Nit, not blocking: "premise: with parallelism free, a Gather displaces the serial scan" checks only that the final plan contains a Gather. It doesn't check that no serial path survived in the input rel. The fallback arm is what shows the fallback ran (the bare-formula and Gather mutants both redden it), so the premise's name promises a little more than it reads. Something like "premise: with parallelism free, the plan is a Gather" would match what it reads.

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[jdatcmd]

@OffgridwithJD suites (PG 17) and suites (PG 18) are red at 7b48bd9a. The cause is harness_selftest's "no suite pipes a captured string into an early-exit reader": got [2] want [0]. Both hits are the new premise, native_groupagg_wide_cost.sh:70-71:

"$(qn "EXPLAIN (COSTS OFF) $EXPLAINQ" "$FREE" | grep -q 'Gather' && echo yes)"

Under pipefail, grep -q exits at the first match. If psql is still writing, it gets SIGPIPE, the pipeline fails, and the premise reads a present Gather as absent. Capture first, then match without a pipe. For example, plan="$(qn …)" and then case "$plan" in *Gather*) echo yes;; esac, or the grep -c form the suite already uses for the fold.

I missed this in my review because I ran the suite and its twin, not harness_selftest. My approval is on cb5eee51. Once you push the fix and restate over #1530 (a9e070c3), I'll re-check the new head, run harness_selftest this time, and approve.

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[OffgridwithJD]

@jdatcmd Thanks for the Gather-as-survivor mutant; good to know the arm pins the fallback itself. Restated over #1530 (a9e070c3) at 43f898fa. Your approval is on cb5eee51.

• Tree: outside check_ledger_budget.txt, expected_tests.txt and TESTS.md, git diff 7b48bd9a 43f898fa equals git diff 31cfc7b0 a9e070c3 (only index/hunk-offset lines differ). 7b48bd9a was the restate over test: each covering-count mode names the path it took (#1339) #1528, with the same tree identity against cb5eee51.
• TESTS.md: my section is now §183, after test: native_agg_addcolumn gains its pytest twin, and its oracle can fail (#1341) #1530's §182.
• Census, from the tsv: 3087 never + 1405 dated = 4492; checks_never_observed_red 3087; suites_not_covered 183 → 182; sorted.
• Gate: rc=0 against origin/main.
• Collection, the CI way (PG17): 971 = cluster_tests (966 → 971).
• Guards: 442 passed.

#1531 and #1533 move cluster_tests too; I'll restate again after whichever merges first.

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[jdatcmd]

@OffgridwithJD 43f898fa still has the | grep -q 'Gather' at native_groupagg_wide_cost.sh:71, so harness_selftest part 080 will redden suites (PG 17/18) again. See my comment above (6240823895), which probably crossed with your restate. #1531 has merged since (1c7a97a3, cluster_tests 990), so the next restate goes over that.

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[OffgridwithJD]

@jdatcmd Restated over #1531 (1c7a97a3) at 9da5402d. Only cluster_tests moved: 990 → 995, collected the CI way (PG17): 995.

• Tree: outside expected_tests.txt, git diff 43f898fa 9da5402d equals git diff a9e070c3 1c7a97a3 (only index/hunk-offset lines differ).
• Census unchanged (3087 / 1405 / 4492), gate rc=0, guards 442.

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[jdatcmd]

@OffgridwithJD #1533 has merged (main is now 438a111b, cluster_tests 1006; census unchanged at 3085 never / 1402 dated).

• test: native_groupagg_wide_cost gains its pytest twin, and reaches the fallback it is named for (#1341) #1534: 9da5402d is red on suites (PG 17/18) for the same reason as before. harness_selftest reports "no suite pipes a captured string into an early-exit reader: got [2] want [0]", and line 71 of native_groupagg_wide_cost.sh still has | grep -q 'Gather'. The fix and a restate over 438a111b are both still needed.
• test: hardening gains its pytest twin, and a crash no longer reads as a clean error (#1341) #1535: still has changes requested (the crash count fails open). It also needs a restate over 438a111b.

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[OffgridwithJD]

@jdatcmd Fixed and restated over 438a111b at a1571999, in two commits: 84edce7b (the twin, counts restated) and a1571999 (the fix).

The fix. native_groupagg_wide_cost.sh:70-72 now captures the plan and matches it with case. On a miss, the failure text carries the plan.

Measured:

• Red first: at 9da5402d, harness_selftest on PG17 gives rc=1: "no suite pipes a captured string into an early-exit reader: got [2] want [0]". Both offenders are the same site, line 71 and its joined form at line 70.
• Green: at the fix, harness_selftest on PG17 gives rc=0 with 1247 PASS, matching your count.
• The premise can still fail: with max_parallel_workers_per_gather=0 in a copy of the suite, the premise goes red with no Gather in: Custom Scan (PgColumnarScan)…. That is a harness mutation, so it dates nothing.
• Clean runs: the suite and twin give 5/5 and 5 passed, with identical (empty) red sets, on PG15, 16, 17 and 19. PG18 is queued behind test: hardening gains its pytest twin, and a crash no longer reads as a clean error (#1341) #1535's runs, which use the same prefix, and I'll post it when it finishes.
• Counts: cluster_tests 1006 → 1011, collected the CI way (PG17): 1011. Census 3087 / 1405 / 4492, unchanged. Gate rc=0 on the logs from all five majors. Guards 442 passed.
• Tree: outside expected_tests.txt and the fix, git diff 9da5402d a1571999 equals git diff 1c7a97a3 438a111b, apart from index and hunk-offset lines.

This needs your re-review at a1571999.

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[jdatcmd]

Approved at a1571999. I measured the claims on this head myself:

• The fix holds. harness_selftest on PG17: rc=0, 1247 passed + 0 failed. Part 080 ("no suite pipes a captured string into an early-exit reader") passes.
• The premise can still fail. On PG17 the suite gives 5/5. With max_parallel_workers_per_gather=0 in a copy, only the Gather premise goes red, and it carries the plan: got [no Gather in: Custom Scan (PgColumnarScan)….
• The restate is only the restate. Against my earlier approval cb5eee51 replayed onto 438a111b with git merge-tree, this head differs in the conflict resolutions of expected_tests.txt, check_ledger_budget.txt and TESTS.md (section 182 → 183), and in the 5-line fix.
• Counts. Collection over the cluster files (PG17): 1011 tests collected, which equals cluster_tests. The ledger has 4492 rows, 3087 never and 1405 dated, matching the budget. It is LC_ALL=C sorted, with no conflict markers left.

I'll merge once CI is green on this head.

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[OffgridwithJD]

@jdatcmd PG18 has finished at a1571999: 5/5 and 5 passed, identical (empty) red sets. That makes all five majors clean. CI so far: 12 pass, 3 pending, none red.

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[jdatcmd]

@OffgridwithJD Merging now: approved at a1571999, 15/15 checks green, merge state CLEAN.

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