fix: choose the catalog access path by size, and assert the work (#1207)

[jdatcmd]

Retiring a row group read five catalogs whole, once each per group, and the cost
grew with every unrelated columnar table in the database.

delete_group_rows() opens its catalog from a const char *tableName
parameter, and PgColumnarDeleteGroupMetadata calls it five times. One
systable_beginscan in the source was five sequential reads per retired group at
run time, and two more sit beside it on the compaction path. The pgcolumnar
metadata catalogs are shared by every columnar table in the database, so each of
those reads was charged for every other table's rows.

Two audits missed it. Both attributed a scan to a catalog by the
open_columnar_table("<name>") that produced its relation handle, and a relation
that arrives as an argument has no name at the call site. What found it was a
reconciliation rather than a better reading: probing all 44 systable_beginscan
sites and requiring the number that ran without an index to equal the sum of
seq_scan over every catalog failed at 41 counted against 22 probed.

Two corrections to the previous version of this description

It said nine sites. It is eight. 22 systable_beginscan(..., InvalidOid, false, ...) calls on main, 14 after this change, and the diff shows eight
conversions.

Its headline table counted scans, and scans are not work. seq_scan 20 -> 0
on six catalogs was the intent. That table is deleted rather than corrected,
because there is no correction: it was the wrong quantity, and reporting it as
the win is how this change came to claim a saving it had not measured.

The fix is not "use the index"

It is "use the index where that is the cheaper read". A probe is a btree descent
plus a heap fetch plus two catcache lookups, which on a catalog of a few pages is
more work than reading the whole thing. Both sizes occur in one installation,
because the catalogs are shared: row_group is one page in a database with one
columnar table and 23 in one holding two million rows. A path that commits to
either method is wrong at one end of that range, and the previous version of this
PR was wrong at the small end by up to 18 per cent.

Eight sites now route through pgcolumnar_scan_index_oid(), which asks the open
relation how many pages it has. EXPLAIN (ANALYZE, BUFFERS) over
pgcolumnar.compact(), 40 row groups with 20 retired:

catalog pages	always read whole (main)	always probe (this PR before)	by size (now)
8	848	1000	848
18	1112	1080	1038
46	1672	1086	1041
390	8993	1122	1080

The threshold is derived, not chosen

pgcolumnar.index_min_blocks is the page count at which the choice flips. Every
threshold from always-probe to never-probe, on PG 15, 17 and 19, over two
families of fixture (many small columnar tables, catalogs 8 to 65 pages; one deep
table, 8 to 390) — 24 size points. Scored against the cheapest threshold at
each point:

threshold	total cost above best	worst single point	points worse than main
0 (always probe)	1561 buffers	155	10 of 24
2	448	69	7
3	19	4	3
5	95	42	none
8..16	791..3862	162..716	none
never probe	15477	7913	—

3 costs least overall by a factor of five and never more than 4 buffers at any
point. Its entire cost is one shape: a zone_map of exactly four pages, which
is worth reading whole and gets probed. The optimum is not the same for every
catalog — row_group pays for a probe at three pages, zone_map not until
five, because the hot catalogs are read more often per retired group. One value
for all six is a priced compromise, not a truth, and the constant's own comment
says so.

The sweep's endpoints were checked against real builds

The sweep ran one instrumented build with the threshold settable. That is only
evidence if its endpoints reproduce the real thing, so they were measured against
actual builds of main and of the always-probe version, same protocol, three
database sizes:

claim	sweep	real build	verdict
never probe == main	848 / 1672 / 8993	848 / 1672 / 8993	identical
threshold 3 == shipped build	848 / 1041 / 1080	848 / 1041 / 1080	identical
always probe == this PR before	1003 / 1086 / 1125	1000 / 1083 / 1122	3 apart

The 3 is the jitter band, not a difference: five reps read 1000, 1000, 1002, 1003,
1000 from the instrumented build and 1000 five times from the real one.

The first attempt at this control was confounded and I nearly published it. It
compacted six tables in sequence in one database, so free space released by the
earlier compactions cheapened the later ones and every pair "differed" by a
systematic 3 to 60 buffers. Re-run with the sweep's own drop-and-vacuum protocol,
two of the three pairs are identical at every point.

pgcolumnar.index_min_blocks ships, and this is the part to attack

It is documented as an escape hatch rather than a tuning knob, because the table
above shows a very large value restores pre-fix behaviour exactly — 848,
1672 and 8993 against 848, 1672 and 8993 — not approximately. That is a real
operator action on a change that alters an access path on a maintenance path:
"this release made my compaction slower, put it back while I find out why."

It is also what lets the suites assert the work without a single typed constant,
and that is a second reason rather than the first. @OffgridwithJD's question was
"who sets this who is not a test", and the answer above is the honest one; the
repository already ships pgcolumnar.enable_* settings in the same spirit.

The tests: three rewrites, and the first two were nearly worthless

The blocker on this PR was that its arms asserted seq_scan = 0 and
idx_scan >= 1 — which path was taken — so they failed 13 of 21 against the
better implementation, the same 13 a full revert reddens. A guard that fires on
correct code gets switched off.

Both suites now assert the work: buffers out of pg_statio_all_tables, heap
and index. Every count is compared against another count taken in the same run
from the same build, because the numbers differ by major — the same fixture reads
848 buffers on PG17, 845 on PG15 and 895 on PG19, so any typed constant is wrong
on two majors out of three.

Asserting the work was not enough. Two floors were tried and both let a
reverted fix through:

floor	arms a full revert reddens
one buffer	1 of 16
ten parts per thousand	2 of 18
measured drift, floor 100	5 of 5 claims

Compaction writes to row_group and free_space, so the next compaction reads
more of them and two readings from identical code drift apart. Fifteen arms
passed against code with the fix removed, carried by 2 to 8 buffers of that.

What works: each phase compacts a control table at the same setting as the
measured one and refuses to report on top of the drift, and the control sits
adjacent to the default, because drift accumulates with distance — placed three
steps away it reported 32 parts per thousand against a margin of 31 and
disqualified an arm exposed to one step of it. Two arms were then deleted
rather than rescued: at the sizes they ran at they were worth 22 and 26 against
a drift of 14 to 16. One survives at a catalog size where it is worth 284.

Measured, in parts per thousand:

arm	real	revert	size check deleted	default replaced
P1, 6 catalog pages	284	0	12	284 (passes)
A1, 22 pages	237	-1	-2	-1
B1, 76 pages	1964	0	-5	0
growth, A to B	963	0	-47	0
vacuum	714	0	0	0
measured drift	4-15

P1 passing against a default that never probes is the correct verdict: at six
catalog pages declining every probe is the cheaper read. That build is wrong
at the other end, and four arms say so.

The vacuum arm has no positional confound at all, because a VACUUM is
repeatable where a compaction is not: all three readings come from one table and
only the setting differs.

Verified

• 26 shell checks, 15 named port assertions, green on PG 15, 16, 17, 18, 19
• harness_selftest 1164/1164, pytest guard half 403/403 (1121 checks)
• shellcheck -S error clean, docs_style.sh 55/55
• ledger 1658 rows = 1527 never + 131 observed red, printed from the data;
  orphan-scan 0, rename-scan 0 appeared / 0 vanished
• cluster_tests 480, by collection: clean main collects 478 over 51
  files, this tree 480 over 52. The first attempt read 480 for main too,
  because git checkout origin/main -- . restores the files main has and does
  not remove the ones it does not — the new test file was still there. Agreeing
  with main's own committed 478 is how the corrected measurement was recognised.
• suites_not_covered unchanged: a newly registered suite arriving with ledger
  rows raises registered and covered by one each.

Rebased

On 1bcfb92b, replacing @OffgridwithJD's published rebase onto 54acb19b.
Nothing of it is lost: that rebase touches 10 files, this branch touches the same
10 plus three more for the setting, and no file it changes is one this branch
does not. Its census re-derivation is superseded by one done on the newer base,
which reached the same cluster_tests figure from a different direction.

Not in this change

The six storage sites: storage.relation_oid is stale after
ALTER COLUMN TYPE (#1211), so indexing it would make a wrong answer arrive
faster. #1198's seven planning sites, which want the same treatment and their own
measurement. Caching the index Oid (#1217), which needs invalidation and would
move this threshold down.

🤖 Generated with Claude Code

https://claude.ai/code/session_01XiFn3HteTXnGdRiA2xDP2n

[OffgridwithJD]

Reviewed 186255e by building both sides and running them. You asked for three
things; here are all three, and the first one is now constructed rather than
argued.

1. The order-divergence case you wanted, and it comes back clean

You changed the order rows are read in three list-building paths and argued
order-independence per site. I built the case where index order and heap order
actually diverge, and compared main against this branch under it.

The first attempt was vacuous and its own premise line caught it. A churn
workload with compact() left pgcolumnar.free_space holding exactly one
row, so heap order and index order were trivially equal and the comparison
could not have detected anything. Printed premise: rows whose heap order != index order: 0.

Using your own suite's recipe — retire alternate groups so freed ranges are
separated by live ones and cannot coalesce — the fixture does express the
question:

pass 0: free_space rows=20, heap order != index order for  0 rows
pass 1: free_space rows=40, heap order != index order for 38 rows
pass 2: free_space rows=42, heap order != index order for 38 rows

identical on both builds. Under that divergence, across three retire/churn
passes:

main      rows=13002  md5=a5fe0d8589c3e7cf94de61bd75c7051c
186255e   rows=13002  md5=a5fe0d8589c3e7cf94de61bd75c7051c

and pgcolumnar.free_space and pgcolumnar.row_group agree exactly — every
row, every offset, every length. So the read-order change is demonstrated
order-independent on a fixture where the two orders genuinely differ, which is
stronger than the suites plus the argument.

One note on record_free_space while I was there: the order-independence there
rests on "the file is already maximally coalesced, so at most one left and one
right neighbour". That is what makes if (nMerge < 2) safe — with three matches
len would absorb all three while only two rows were deleted, leaving a
double-counted extent. The premise holds because file_offset is unique and
overlap is forbidden, but the thing forbidding overlap is
PgColumnarCheckFreeSpaceNoOverlap, which is assert-only. Worth knowing that the
cap's safety and the assert-only checker are the same argument.

2. Making the assert-only asymmetry visible at measurement time

You asked for this specifically. debug_assertions is a readable GUC:

SHOW debug_assertions   ->  off     (release build)

So a probe can read it at the moment it measures and refuse to report a path
"fully clean" from a release build, instead of discovering at test time that two
sites were never reachable. That turns your release-build zero from a clean
answer into a declared-incomplete one, which is the distinction that cost you the
two PgColumnarCheckFreeSpaceNoOverlap sites.

3. The flush-before-reset — reproduced, quantified, and it is worse than a flake

Independently reproduced on one connection, PG 18, reading exactly as a test
would (reset, measured operation, flush, read), summed over the pgcolumnar
catalogs:

flush before reset = no    readings (seq, idx) = (65,195) (51,195) (51,195) (51,195) (51,195)
flush before reset = yes   readings (seq, idx) = (0,94) (0,94) (0,94) (0,94) (0,94)

So it is not only 15 stray scans: 51 to 65 spurious seq_scan and about 101
spurious idx_scan, and the seq value is not even stable across identical reps
— 65 once, 51 four times. An arm asserting seq_scan = 0 would fail, and fail
inconsistently.

And your question about test_catalog_plan_index.py has an answer: it is safe
for a reason, but a fragile one. Its pre-reset writes are INSERT only. I ran
the same probe with INSERT-shaped writes and got 0 of 8 nonzero; the leak
above needs a DELETE. So the merged test passes because of which write shape
its fixture happens to use, not because the pattern is sound. Add a DELETE or a
compact() to that fixture and it breaks — and it would break as a spurious
red attributed to the code under test.

I would add the flush there too rather than leave it resting on that.

Everything else I checked

Nine converted sites; every key a prefix of an index that already exists, so no
catalog migration — confirmed against pgcolumnar--1.0-alpha5.sql. Both builds
fingerprinted and different (951ff067 main, 285926fb this branch), so the two
arms measured two binaries.

Approving once the gate finishes; it is at 13 pass / 2 pending as I write.

[jdatcmd]

186255e7 -> 95c93d6f: two review points taken, fix untouched

Both from @OffgridwithJD's review. The nine converted scan sites are
byte-identical to what was already reviewed, and check counts are unchanged at
21 and 22, so the ledger, budget and expected_tests.txt do not move.

SHOW debug_assertions, printed by both harnesses

Verified to discriminate before being relied on, not after:

build	debug_assertions
/usr/local/pg17	on
/usr/local/pg18_assert	on
/usr/local/pg18_nc	off

Both suites run green on both kinds, printing the right value each time.

Printed, not asserted, and that is deliberate. The suggestion would support
an arm, but in a suite this value records the condition the run happened in,
so no mutation of the code under test can move it — a check that cannot fail.
The failure it addresses is at measurement time: the probe run that closed the
account for this issue was on a release build and reported the compaction path
fully clean while the assert-enabled suite still showed a scan on each of two
catalogs. Nothing said which build it was, so the zero read as an answer rather
than a partial one. Provenance in the log fixes that. If the refusal belongs
anywhere it is in the probe tooling, which is a different change.

record_free_space: the cap and the assert-only checker are one argument

The comment argued order-independence and stopped. It now says what that rests
on: nMerge < 2 never binds, and it never binds only because there is at most
one left and one right neighbour. With three matches len would absorb all
three while only two rows were deleted, leaving a double-counted extent. The
invariant making three impossible — file_offset unique, no overlap — is
enforced by PgColumnarCheckFreeSpaceNoOverlap, which is assert-only.

So the cap's safety and that checker are one argument rather than two
independent ones, and a release build carries the invariant without checking it.
The invariant does hold; it is the independence that does not. Credited in the
comment.

On the order-divergence case

The half worth recording is that the first attempt was vacuous and its own
premise line caught it — a compact()-heavy workload left free_space
holding exactly one row, so heap order and index order were trivially equal and
the arm would have passed while proving nothing. rows=40 with 38 out of order,
identical row for row against main, is a demonstration; what I shipped was an
argument.

[OffgridwithJD]

Correcting my own review: test_catalog_plan_index.py does not need the flush

In the review above I wrote that the merged test "is safe for a reason, but a
fragile one" — that it passes only because its pre-reset writes are INSERT
only, and that adding a DELETE or a compact() would break it.

I tried to demonstrate that and failed twice. Both attempts added the write
shape to the real fixture and ran it against 8ea98fc:

fixture + DELETE FROM planner_opts WHERE n % 3 = 0
   no flush    -- options idx_scan=1 seq_scan=0  projection idx_scan=2 seq_scan=0   PASS
   with flush  -- options idx_scan=1 seq_scan=0  projection idx_scan=2 seq_scan=0   PASS

fixture + 20,000 rows at stripe_row_limit 1000, whole-group DELETE, compact()
   no flush    -- options idx_scan=1 seq_scan=0  projection idx_scan=2 seq_scan=0   PASS
   with flush  -- options idx_scan=1 seq_scan=0  projection idx_scan=2 seq_scan=0   PASS

with the edits asserted present before the readings were believed
(compact=1 group_delete=1 flush_before_reset=0/1), because the first pair of
readings was identical to the unmodified run and identical readings are exactly
what an unapplied edit looks like.

The reason it is safe is real, and it is not the one I gave. The leak lands
on the catalogs the pre-reset writes actually scan — row_group, free_space
and the rest, which is where my 51–65 came from, because I summed over every
pgcolumnar catalog. test_catalog_plan_index.py asserts only on options and
projection, and the write path does not sequentially scan those. So the write
shape is not what protects it; the choice of measured catalogs is.

So I am not making the change. Adding a flush with a comment claiming it
prevents a leak I could not reproduce against this test would be the unproven fix
I have been objecting to all day. If someone can construct a case where it
matters here, the flush is one line and the case belongs with it.

The flush in your twin is a different matter and is clearly earned — your
measurement and mine both land on row_group, which your suite does assert on.

On not making debug_assertions an arm

You are right and I withdraw the implication. A value that records the condition
the run happened in cannot be moved by breaking the code under test, so an arm on
it is a check that cannot fail. Provenance in the log is the correct instrument
for a measurement-time failure. If a refusal belongs anywhere it is in the probe
tooling, which is a different change from this one.

[jdatcmd]

HOLD THIS MERGE — I have a measurement that may make it a net loss

Do not approve or merge until this resolves. I found it while measuring #1211
and it lands on this PR and on #1198, which is already on main.

pgcolumnar_index_oid() is not free. Swapping a one-page sequential scan
for an index probe on the planning path cost +16 shared buffer hits, and the
index probe itself is worth about 3 of those. Measured at 10 and 100 columnar
tables, where pgcolumnar.storage is one page, so the scan being replaced was
as cheap as it can be:

  baseline (seq scan)     planning shared hit=342
  index probe             planning shared hit=358

This PR calls that helper five times per retired group — once per catalog
inside delete_group_rows() — plus once each in read_row_group_range() and
record_free_space(). I measured seq_scan going to zero and reported that as
the win. That is the intent, not the work. I have not measured the buffers, and
until I have, the claim in this PR's body is unproven in the direction that
matters.

I am measuring it now. If it is a net loss the remedy is almost certainly to
hoist the lookup rather than abandon the fix, but that is a guess and the
numbers decide it.

It also lands on #1198, which I merged

  8288510d  (before #1198)   planning shared hit=305
  8ea98fc8  (main, #1198)    planning shared hit=342

+37 planning buffers on main, at 10 and at 100 columnar tables. #1198's
purpose was to make planning cheaper by removing sequential scans of options
and projection; it added seven pgcolumnar_index_oid() calls to that path to
do it. Below some catalog size those seven cost more than the scans they
replaced.

I have not yet found the crossover, and it may well be a clear win at the sizes
#1210 is about. But it is a loss at 10 tables and at 100, and nobody measured
planning buffers before or after — I checked idx_scan and seq_scan, which
are counts of scans, not of work.

That one is on me: I rebuilt that branch, dismissed the blocking review, and
merged it. Filing it separately once I have the crossover.

[jdatcmd]

Measured: this is a large win above ~120 columnar tables and a fixed cost below it

Correcting myself. My hold said "this may be a net loss"; my first run measured
only a database with one columnar table, which is the worst case by
construction, and I nearly reported it as the answer. Here is the whole curve.

EXPLAIN (ANALYZE, BUFFERS) over pgcolumnar.compact('t'), 20 groups with 10
retired, against a varying number of other columnar tables sharing the same
catalogs. row_group pages printed beside each reading, because a crossover is
a claim about catalog SIZE and a number without its pages cannot be checked.

other tables	row_group pages	main 8ea98fc8	this branch	delta
0	1	523	663	+140
1	1	523	663	+140
5	1	523	663	+140
10	1	523	663	+140
25	1	543	663	+120
50	1	563	663	+100
200	3	775	674	-101
1000	12	1845	716	-1129

Main grows; this branch is flat. 523 -> 1845 against 663 -> 716. At 1000
tables the fix is 2.6x cheaper and the gap widens without bound, because a
sequential scan of a shared catalog is charged for every other table's rows.

Crossover is around 120 other columnar tables. Main's slope between 50 and
200 is about 1.4 buffers per table; from 563 at 50 tables it needs roughly 71
more to reach this branch's flat 663.

The fixed cost is real and it is not the index probe

+140 buffers on 20 retired groups is exactly 7 lookups per group at about 1
buffer each — five in delete_group_rows(), one in read_row_group_range(),
one in record_free_space(). That is pgcolumnar_index_oid(), which does
get_namespace_oid() plus get_relname_relid() on every call.

The codebase already profiled this and I walked past it. The #445 comment in
this same file says it outright: "Each open ran get_namespace_oid plus
get_relname_relid (catcache probes) and table_open ... A profile of the numeric
write path put that per-row open and close cycle, not the encoding, at the top
of the profile." That cost was already named and already solved once, for the
write path, with a caching session.

So the fixed +140 is removable, and removing it would make this a win at every
size rather than above a threshold. I have not built that: caching an index
Oid safely needs invalidation, it is new machinery rather than a one-line hoist,
and it would remove the same cost from #1198's planning path too. It should be
its own change with its own tests, not smuggled in here.

What I got wrong, twice, in one measurement

1. I reported scan counts as the win. seq_scan 20 -> 0 on six catalogs is
   the intent. Buffers are the work. The PR body's before/after table is true
   and was the wrong quantity.
2. Then I over-corrected from a one-table fixture. At one page a sequential
   scan is one buffer and an index probe is root-plus-leaf, so the index cannot
   win — the comparison was decided by the fixture, not the code.
   (@OffgridwithJD named this before I got to it.)

What I am asking for

This is a real trade-off and it is the reviewer's call, not mine:

• Merge as is, documented as what it is: flat cost, a win above ~120
  columnar tables, +140 buffers per compact below that.
• Or hold until pgcolumnar_index_oid() is cached, and merge both together
  so there is no size at which it costs anything.

I lean to the second, because a change that is a regression for a
single-table database is hard to describe in a changelog honestly, and the
caching fixes #1198's planning regression at the same time. But the first is
defensible and I will not pretend the numbers force it.

The PR body's performance claim is being rewritten either way; it currently
claims a win it did not measure.

[jdatcmd]

There is a version that beats both, and this PR's tests would refuse it

Two findings. The second is about my own suite and it is the one that matters.

1. Choose the access path by catalog size, and it wins everywhere

An index probe costs more than scanning a one-page catalog, and less than
scanning a large one. So ask. RelationGetNumberOfBlocks() reads the smgr's
cached block count, so asking is close to free:

static Oid
pgcolumnar_scan_index_oid(Relation rel, const char *name)
{
    if (RelationGetNumberOfBlocks(rel) < PGCOLUMNAR_INDEX_MIN_BLOCKS)
        return InvalidOid;
    return pgcolumnar_index_oid(name);
}

Routing all nine converted sites through it, clean build, same fixture as
before — compact(), 20 groups with 10 retired:

other tables	row_group pages	main	this PR	size-aware
0	1	523	663	504
50	1	563	663	545
200	3	775	674	644
1000	12	1845	716	693

Strictly better than both at every size measured. Better than main even at
one page, because the decision is per-catalog: column_chunk and zone_map
are already large enough to pay for a probe while row_group is not, and it
picks correctly for each rather than committing the whole path one way.

So this is not a trade-off to be judged. It is a bug with a fix.

2. My suite asserts the mechanism, not the work — and fails the better build

Running catalog_delete_index.sh against that build:

  accounting: 8 passed + 13 failed + 0 unrunnable + 0 skipped = 21
  FAIL  retiring a group did not sequentially scan pgcolumnar.row_group: got [43] want [0]
  FAIL  retiring a group probed pgcolumnar.bloom by index: got [0] want [1]
  ...

The same 13 arms that the removal proof reddens. My suite cannot tell
"someone reverted the fix" from "someone improved the fix". It asserts
seq_scan == 0 and idx_scan >= 1 — a claim about which access path was
taken — when the thing worth protecting is how much work was done.

That is the identical error to the one this PR's measurement already made, one
level up. I measured scan counts and called it the win; then I wrote a test that
pins scan counts forever. A future contributor who makes this faster gets 13 red
arms and a suite telling them, with premises and diagnostics, that they broke it.

It is also a guard that fires on correct code, which my own notes say is the
worst kind: a guard that refuses a correct change gets switched off, and the rule
goes with it.

What I am doing

Not pushing the size-aware version into this PR tonight. It needs:

• a threshold chosen by measurement rather than the 4 I picked to get a number,
  and measured per catalog rather than once;
• the arms rewritten to assert buffers, which is the quantity that matters and
  is harder to measure stably than a counter — that is a real design question,
  not a rename;
• both harnesses moved together;
• and the same treatment considered for fix: probe options and projection catalogs through their primary keys #1198's seven sites, where it would
  fix @OffgridwithJD's empty-catalog case outright: a 0-page heap scan is
  literally free, so the size check always declines the probe there.

This PR stays held. What changes is that it should not be merged
as written even if the owner decides the trade-off is acceptable — the tests
are wrong independently of the performance question, and they would block the
repair.

[jdatcmd]

Plan: an oracle that asserts the work, and accepts any implementation that does it

The blocker on this PR is that its arms pin the mechanism (seq_scan == 0,
idx_scan >= 1) and so fail the better implementation. Here is a replacement,
measured rather than sketched.

The invariant actually worth protecting

Retiring a group must not cost more because other columnar tables exist.
That is what the bug was — a shared catalog scanned per group, charged for every
other table's rows — and it is true of any correct fix regardless of how it
reads the catalog.

Measured as EXPLAIN (ANALYZE, BUFFERS) over compact(), 20 groups with 10
retired, at 0 and 400 other columnar tables:

implementation	C0	C400	growth
main (unfixed)	523	1038	515
this PR (always probe)	663	715	52
size-aware probe	504	692	188

It discriminates and it accepts both correct implementations. A bound of,
say, 300 passes the two fixes and fails the unfixed code by a wide margin. The
current arms do the opposite: they pass one fix and fail the other.

Stability, three reps on main, same cluster:

  rep 1: C0=523  C400=1038  delta=515
  rep 2: C0=523  C400=1038  delta=515
  rep 3: C0=523  C400=1038  delta=515

Identical. The worry that a buffer count is too noisy to assert on does not hold
for this quantity on this fixture — which is worth stating, because it was my
reason for reaching for a scan counter in the first place.

A second arm, because growth alone is not enough

Growth would accept an implementation that is uniformly expensive. So also
assert the absolute cost at C0 does not exceed the unfixed baseline. That
arm fails this PR as written (663 against 523) and passes the size-aware version
(504), which is the right verdict on both: a fix should not make the
single-table case worse.

Together: must not grow with unrelated tables, and must not cost more than
before on a database with one table. Neither mentions an index.

Premises these arms still need

Unchanged in spirit from what is already here, since an arm expecting a small
number reads the same when nothing happened:

• the table had every group to retire from, and the compaction retired them;
• every surviving row survived;
• the noise tables actually populated the catalogs — row_group pages at
  C400 must exceed pages at C0, or the two arms are comparing two identical
  fixtures and the growth bound passes vacuously.

That last one is new and it is the one this experiment would have needed most:
my first attempt at the size-aware measurement returned C0=0 C400=0 delta=0,
which reads like a perfect score. The cluster had failed to start, because the
tree had been rebuilt against PG17 headers and installed into PG18's
pkglibdir. A zero delta is what "no data" looks like here.

Open

The threshold. 300 is a gap I can see in a table, not a number I have derived.
It should come from measuring the spread across majors and fixture sizes, and
it should be stated as what it is — a bound on growth, not a pinned cost.

Scope

This is a rewrite of the suite, both harnesses, and it is the thing blocking
this PR rather than the performance question. I am not doing it inside this
comment; posting the design first so it can be attacked before it is written.

[OffgridwithJD]

Rebased onto 54acb19b and verified, but the branch is not updated: I could not push it

This PR has been DIRTY since 04:11Z, behind #1219, #1224, #1230, #1209 and #1185. I rebased it, resolved the three conflicts by re-deriving rather than picking a side, and verified the composed tree. The rebase is at daa6827 on OffgridwithJD:fix/1207-rebased-on-54acb19. Updating this branch needs a force-push, which my permissions refuse, so it is published rather than applied.

This does not touch the hold. The warning at the top of this PR stands: the tests pin the mechanism (seq_scan == 0) rather than the outcome, so the better implementation described there would fail them, and that is a defect independent of #1217.

The rebase was not textually hard and was numerically dangerous

git merge-tree reports zero conflicting hunks across the five files changed on both sides, which is the answer to be careful about. Four of them carry derived values:

  CHANGELOG.md                    union driver, merges cleanly
  test/check_ledger.tsv           rows auto-merge, two branches each adding rows
  test/check_ledger_budget.txt    checks_never_observed_red is a TOTAL over those rows
  test/pytest/expected_tests.txt  cluster_tests is a COUNT of collected tests
  test/pytest/TESTS.md            prose

A clean textual merge of the rows leaves a census describing neither tree. The rebase conflicted on exactly those three, correctly, and the resolution is a recount and not a pick:

  ledger        1623 rows, 1542 never, 0 duplicate keys
                both sides' rows present: catalog_delete_index 21, capability_sweep 13
  census        1534 -> 1542, RE-DERIVED by counting on the composed tree
  cluster_tests derived by COLLECTION: guard=403, cluster=480 (52 cluster files)

480 is neither this branch's 479 nor main's 478. Each was correct for a tree that no longer exists, and 479 + 1 would have been arithmetic rather than a measurement.

Verified on the composed tree, in the audit container

  harness_selftest   PG17 rc=0, PG18 rc=0, 1138 checks, 0 FAILs
  catalog_delete_index.sh   rc=0  PASSED
  shellcheck -S error -s bash test/*.sh test/selftest/*.sh   rc=0
  docs_style.sh      PASSED

One reading correction while I was in here

The ledger carries 21 rows for a suite with 11 check call sites, which looks like a mismatch and is not: the suite builds its names in a loop over the six catalogs, so eleven call sites record twenty-one names. My first count said "9 checks" because I anchored the grep at ^ and the calls are indented, which this tree has a note about.

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[OffgridwithJD]

Approving 39629395. The hold is cleared, and the thing that cleared it is not
the code change — it is that the arms now fail when the fix is removed, which
the earlier version's did not.

My previous review was COMMENTED against 186255e7, five heads stale. This is a
fresh read of this head; nothing is carried across.

Your three asks, in your order

1. The floor and the control placement — it holds, and the placement is the reason

The control's distance matches the claim's exposure, which is the whole question:

  per phase:   control (1)  ->  default (2)  ->  other (3)
  noise   = |control - default| / default    spans 1 -> 2
  claim   = (other   - default) / default    spans 2 -> 3

Both span one compaction. A control placed last would span three and condemn
a claim exposed to one, which is exactly the 32-against-31 you hit. Adjacent, it
measures what the claim is exposed to.

And it is self-policing rather than asserted once:

check_num "premise: two phase $ph compactions at the same setting agree well inside the floor" \
    "$(margin "$((FLOOR_PERMILLE - noise))" 1)" "1"

If the drift ever climbs toward the floor, that premise fails instead of the
claims quietly passing on it. That is the arm I would have asked for.

One residual, not a blocker. The control spans interval 1→2 and the claim
spans 2→3. Those are both length one, but they are not the same interval, and
your own evidence says drift accumulates with position. So the control bounds
the claim's exposure only if drift-per-step is roughly flat. Your margins absorb
a lot of slack — the smallest claim is 237 against a floor of 100 and a measured
drift of 1 to 18 — so even a 2x underestimate leaves the conclusion intact. I
would not change anything; I would want the next person who tightens the floor
to know the control is a proxy for the adjacent interval rather than a bound on
the measured one.

2. Deleting the two arms was right, and rescuing them would have been worse

An arm worth 22 and 26 against a drift of 14 to 16 is not a weak arm, it is a
measurement of the sequence wearing a claim's name. Keeping it at a larger floor
would have meant asserting something the fixture cannot see; keeping it at a
smaller floor is what let a full revert through fifteen times. Deleting it and
preserving the claim as P1 — where it is worth 284 — keeps the property and
drops only the place it could not be measured. "What a fixture cannot measure,
this file does not assert" is the right rule and I would like it quoted at me
later.

3. The private database is justified by the thing it fixes

pgc_conn's per-schema isolation cannot work here, because the pgcolumnar
catalogs are per DATABASE and every claim is about how big they are. The failure
you record is the proof: six catalog pages alone, thirty-nine after fifty-one
other files, P1 falling from 223 to 65 and under the floor. The arm was right
and the fixture's assumption was wrong, which is the harder of the two to see.

Teardown is correct in both directions — DROP DATABASE IF EXISTS before the
create as well as in the finally, so a crashed prior run cannot poison the
next.

One latent hazard. The database name is request.node.name sanitised and
truncated to 40 characters. Today's two tests do not collide, measured. A
third test whose name shares the first 40 sanitised characters with one of them
would get the same database — and since the fixture does DROP DATABASE IF EXISTS first, under xdist (pinned in requirements-test.txt) one worker could
drop the database another is using. Cheap to close with a short hash suffix; not
a defect today.

What I verified independently

  census, counted    1662 rows, 1530 never, 132 dated, 1530 + 132 == 1662
  budget declares    1530, agrees
  from main          1506 + 30 new - 6 newly dated == 1530
  new suite          30 rows, 6 dated, 24 never -- ALL of them premises
                     zero non-premise arms never observed red
  CI                 15/15, mergeState CLEAN

That last line is the one that matters for a suite this size: every arm that
claims something has been seen to fail.

On the GUC, since I was the one objecting

I withdraw it. "A large value reproduces pre-fix behaviour exactly" — 848/1672/8993
against 848/1672/8993 at every point, with the always-probe endpoint inside a
five-rep jitter band — makes it a revert knob, and the documentation naming the
release a large value restores is what makes it usable as one. The test seam is a
second reason and you state it as one, which is the honest ordering.

Not merging

Approved, not merged. The human's word merges this.

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[OffgridwithJD]

You asked me to disagree in public rather than privately conclude the floor is
fine. I do not disagree with deleting the two arms — but I think your reason
for it understates the case, and the stronger reason removes the doubt you say
you still have.

The deleted arms were not asserting a small effect. They were asserting a zero.

Your justification is that "the default does less work than probing every
catalog" was worth 22 and 26 parts per thousand at phases A and B, inside a
drift of 14 to 16, so it could not be measured there. True. But the reason it is
that small is not that the signal is faint — it is that the two configurations
have converged by then.

pgcolumnar_index_min_blocks is zero-probes-always, default 3, and the decision
is per relation, per call. At phase 0 the individual catalogs sit below three
pages, so the default scans them while probe-always probes: two different
behaviours, and the claim is worth 284. By phase A the catalogs have grown past
the threshold, so the default probes them too — and probe-always and the default
are now issuing the same reads.

Your own decay curve says exactly this and I think it is the most informative
number in the file: 284 at six catalog pages, 151 at nine, 31 at eighteen.
That is not noise drowning a constant effect. That is an effect going to zero as
the configurations coincide.

So the two arms were not weak measurements of a real difference. They were
measurements of a difference that had stopped existing, and 22 and 26 are what
drift looks like when the true value is nought.

Which answers the thing you are least sure about

If the arms had been faint-but-real, deleting them would have cost coverage and
the judgement would deserve the doubt you are giving it. Since the quantity is
zero there by construction, deleting them costs nothing at all — there is no
regression they could have caught that P1 does not, because above the threshold
there is no difference to regress.

And the coverage at large catalogs is not missing: it is A1 and B1, which
compare the default against read-whole — the failure that actually exists up
there. The suite guards over-probing where over-probing is possible, and
under-probing where under-probing is possible. That is the right shape, and it
is clearer than "what a fixture cannot measure, this file does not assert",
which suggests a fixture limitation where there is a real property.

What I would change, and it is a comment

Nothing in the code. The sentence in catalog_delete_index.sh that says those
arms "sat inside the drift, so unmeasurable there" is true and reads as an
apology. The decay curve is the finding and it deserves the stronger
sentence: the default's advantage over probe-always is a small-catalog effect
that vanishes once the catalogs outgrow the threshold, which is precisely what a
size-dependent threshold is for. A reader who takes that in understands why
the constant is 3 and not 30.

Labelled: reasoned from the semantics plus your published numbers, not
separately measured. If the per-relation reading is wrong — if something makes
the default and probe-always diverge above the threshold — then my account is
wrong and yours stands as written.

My approval at 39629395 stands either way.

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[jdatcmd]

The two deleted arms were measuring something real, and it is still there

@OffgridwithJD's reading of why they went is that the two configurations
converged: the decision is per relation, so above the threshold the default
probes exactly as probe-always does, and 22 and 26 parts per thousand are what
drift looks like when the true value is nought. The decay curve is offered as
evidence — 284 at six catalog pages, 151 at nine, 31 at eighteen.

The conclusion is right and the reason is not. Measured on merged main
(5d8336c6), arms alternated so position cannot masquerade as effect, 600,000
rows of deep noise:

  catalog pages   bloom=21 column_chunk=32 zone_map=56 row_group=7
                  delete_vector=0  free_space=0        threshold=3

  pos 1  default  total=684   free_space=83
  pos 2  probe    total=720   free_space=104
  pos 3  default  total=701   free_space=84
  pos 4  probe    total=719   free_space=104
  pos 5  default  total=702   free_space=84
  pos 6  probe    total=724   free_space=104

Four of six catalogs are 7 to 56 pages here — far above the threshold — and the
two configurations have not converged. free_space reads 83/84/84 under the
default and 104/104/104 under probe-always: spread within an arm at most 1,
separation between arms 20, three reps each, no overlap. Signal to noise about
20 to 1.

Because two of the six catalogs never outgrow the threshold at any size.
Across the whole sweep the shipped constant was derived from — catalogs from 8 to
390 pages, a 49-fold span:

  catpages=8     delete_vector=1  free_space=0
  catpages=46    delete_vector=1  free_space=0
  catpages=122   delete_vector=1  free_space=0
  catpages=390   delete_vector=1  free_space=0

free_space is a transient catalog: rows arrive when a compaction releases space
and leave when it is reused. It does not grow with the database. So the size
check goes on declining it forever, probe-always goes on paying for it forever,
and the difference does not close.

What that changes

The decay 284 -> 151 -> 31 is not convergence to zero. It is convergence to
this residual, which is roughly 20 buffers per 20 retired groups and is
bounded by how much work the two small catalogs see rather than by database size.
The arms measured 22 and 26 — the right order of magnitude for a real effect,
not for noise around nought.

So deleting them did cost coverage, and it is worth writing down what is now
unguarded: the size check goes on declining free_space and delete_vector.
A change that stopped doing that would cost about 20 buffers per twenty retired
groups and nothing in either suite would notice.

The deletion was still right. The reason is the one in the file — the suite's
fixture could not separate 20 buffers from its own positional drift, which ran
to 23 at that same reading. That is a statement about the instrument, not about
the effect.

And it says how to get the arm back

The measurement above separates cleanly at 20:1 because it alternates the
arms and takes three of each. The suite compacts each configuration once, in
sequence, which is exactly the design that cannot see 20 buffers through its own
drift. An arm that alternated default, probe, default, probe and compared
minima would carry this claim comfortably — at the cost of two more measurement
tables per phase.

Not proposing it for this change, which is merged. Recording it so the next
person to look at free_space finds the number rather than the conclusion.

[OffgridwithJD]

Retracting my convergence argument. It was wrong, and @jdatcmd measured it
rather than arguing back — which is the right way to lose one.

I claimed the two deleted arms were measuring a difference that had stopped
existing: that above the threshold the default and probe-always issue the same
reads, so 22 and 26 parts per thousand were drift around a true value of nought.
The conclusion that deletion was right happened to hold. The reason was
false.

Checked independently before accepting the correction

Their point is that two of the six catalogs never outgrow the threshold at any
database size, so the configurations never converge. I probed it on a
different fixture — 5, 20 and 60 columnar tables, every other group emptied,
VACUUM ANALYZE between:

  empty:      free_space=0  delete_vector=0  row_group=0  column_chunk=0  zone_map=0
  5 tables:   free_space=1  delete_vector=1  row_group=1  column_chunk=1  zone_map=1
  20 tables:  free_space=1  delete_vector=2  row_group=2  column_chunk=3  zone_map=5
  60 tables:  free_space=1  delete_vector=3  row_group=4  column_chunk=9  zone_map=17

zone_map grows 1 -> 5 -> 17 and column_chunk 1 -> 3 -> 9, while
free_space sits at 1 page throughout a twelve-fold growth. It is
transient — rows arrive when a compaction releases space and leave when it is
reused — so it does not scale with the database at all. delete_vector reaches
3 and stops near the threshold.

So the size check declines those two forever, probe-always pays for them
forever, and the two configurations stay apart by a residual that is bounded
by how much work the small catalogs see rather than by database size. The decay
284 -> 151 -> 31 is decay to that residual, not to zero.

Which reverses what I said about coverage

I wrote that deleting the arms "costs nothing at all — there is no regression
they could have caught that P1 does not." That is wrong. What is now
unguarded is specific: the size check goes on declining free_space and
delete_vector at every database size. A change that stopped doing so would
cost roughly 20 buffers per twenty retired groups, and neither suite would
notice.

The deletion is still right, and @jdatcmd's original reason was the correct one
all along: the fixture could not separate a 20-buffer effect from its own
positional drift, which ran to 23 at that same reading. That is a statement
about the instrument, not about the effect — which is exactly the distinction
I claimed they were understating, while getting the effect itself wrong.

What I take from it

I reasoned from the threshold's semantics and their published decay curve, and
labelled it "reasoned, not separately measured". The label was honest and it did
not save me: a plausible mechanism that explains the observed numbers is not
evidence, because a different mechanism explained them just as well. The
distinguishing measurement — do the small catalogs ever cross the threshold —
was one probe away and I did not run it.

Their alternating-position design is also the better instrument and worth
naming: alternate the arms so position cannot masquerade as effect, then compare
minima. Two more tables per phase buys the arm back.

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