test: attack nine index_am_support arms (#1236)

CHANGELOG.md

@@ -18,6 +18,35 @@ true until the next version shipped.
 
 ### Changed
 
+- Nine `index_am_support` arms now carry a mutation that reddens them (#1236).
+  No test changed and no code changed.
+
+  Its only dated rows were two premises about `docs/features.md`, reddened by a
+  **docs** mutation, which cannot reach one arm about what an index does. Eight
+  behaviour arms sat `never` beside them -- unattacked rather than vacuous, the
+  same shape as the first batch.
+
+  Two mutations with disjoint predicted sets, both confirmed exactly:
+
+  ```
+    M-1  the build drops two rows in three, so the index misses entries
+         predicted the four "answers its operator" arms, NOT the build arms
+         observed  exactly those four; CREATE INDEX still succeeds
+
+    M-2  the build errors, so CREATE INDEX fails
+         predicted the four "builds an index" arms
+         observed  those four, plus the GiST overlap premise
+  ```
+
+  **M-2 also shows a weakness in the four answer arms.** With no index in
+  existence they all stayed green: the arm compares a count taken with
+  `enable_seqscan = off` against one taken with the custom scan off, and
+  `enable_seqscan` is a preference rather than a prohibition -- so with no index
+  both sides fall back to the same scan and agree. An arm named *answers its
+  operator with the same rows the scan returns* can be satisfied without an index
+  scan happening. Reported on #1236 rather than repaired here, because this
+  change records evidence and alters no test.
+
 - Fifteen `native_join_runtime_filter` arms now carry a mutation that reddens
   them (#1236). No test changed and no code changed: the arms were attacked and
   the ledger records what happened.

test/check_ledger.tsv

@@ -1383,13 +1383,13 @@ harness_selftest	580-a-hand-rebuild-must-record	premise: the hand rebuild tool i
 harness_selftest	580-a-hand-rebuild-must-record	premise: the shim redirects the install away from the real prefix	15;16;17;18;19	2026-09-23	the shim answers --pkglibdir with the real prefix, so the install is not redirected
 harness_selftest	580-a-hand-rebuild-must-record	the hand rebuild tool records what it installed rather than only naming it	15;16;17;18;19	2026-09-23	the record call removed from rebuild.sh, or the comment stripping removed
 harness_selftest	580-a-hand-rebuild-must-record	the recorder refuses when it is not told what was installed	15;16;17;18;19	2026-09-23	the argument guard removed from pgc_record_source_stamp
-index_am_support	index_am_support	btree answers its operator with the same rows the scan returns	15;16;17;18;19	never	-
-index_am_support	index_am_support	btree builds an index over a columnar table	15;16;17;18;19	never	-
-index_am_support	index_am_support	gist answers its operator with the same rows the scan returns	15;16;17;18;19	never	-
-index_am_support	index_am_support	gist builds an index over a columnar table	15;16;17;18;19	never	-
-index_am_support	index_am_support	hash answers its operator with the same rows the scan returns	15;16;17;18;19	never	-
-index_am_support	index_am_support	hash builds an index over a columnar table	15;16;17;18;19	never	-
-index_am_support	index_am_support	premise: an overlap query reaches the row through a GiST index	15;16;17;18;19	never	-
+index_am_support	index_am_support	btree answers its operator with the same rows the scan returns	15;16;17;18;19	2026-09-23	pgcolumnar_index_build_range_scan drops two rows in three, so the index misses entries
+index_am_support	index_am_support	btree builds an index over a columnar table	15;16;17;18;19	2026-09-23	pgcolumnar_index_build_range_scan errors, so CREATE INDEX fails
+index_am_support	index_am_support	gist answers its operator with the same rows the scan returns	15;16;17;18;19	2026-09-23	pgcolumnar_index_build_range_scan drops two rows in three, so the index misses entries
+index_am_support	index_am_support	gist builds an index over a columnar table	15;16;17;18;19	2026-09-23	pgcolumnar_index_build_range_scan errors, so CREATE INDEX fails
+index_am_support	index_am_support	hash answers its operator with the same rows the scan returns	15;16;17;18;19	2026-09-23	pgcolumnar_index_build_range_scan drops two rows in three, so the index misses entries
+index_am_support	index_am_support	hash builds an index over a columnar table	15;16;17;18;19	2026-09-23	pgcolumnar_index_build_range_scan errors, so CREATE INDEX fails
+index_am_support	index_am_support	premise: an overlap query reaches the row through a GiST index	15;16;17;18;19	2026-09-23	pgcolumnar_index_build_range_scan errors, so CREATE INDEX fails
 index_am_support	index_am_support	premise: btree, which the page claims, has a probe in this suite	15;16;17;18;19	never	-
 index_am_support	index_am_support	premise: gist, which the page claims, has a probe in this suite	15;16;17;18;19	never	-
 index_am_support	index_am_support	premise: hash, which the page claims, has a probe in this suite	15;16;17;18;19	never	-
@@ -1401,8 +1401,8 @@ index_am_support	index_am_support	premise: the fixture holds every row	15;16;17;
 index_am_support	index_am_support	premise: the gist predicate matches rows at all	15;16;17;18;19	never	-
 index_am_support	index_am_support	premise: the hash predicate matches rows at all	15;16;17;18;19	never	-
 index_am_support	index_am_support	premise: the spgist predicate matches rows at all	15;16;17;18;19	never	-
-index_am_support	index_am_support	spgist answers its operator with the same rows the scan returns	15;16;17;18;19	never	-
-index_am_support	index_am_support	spgist builds an index over a columnar table	15;16;17;18;19	never	-
+index_am_support	index_am_support	spgist answers its operator with the same rows the scan returns	15;16;17;18;19	2026-09-23	pgcolumnar_index_build_range_scan drops two rows in three, so the index misses entries
+index_am_support	index_am_support	spgist builds an index over a columnar table	15;16;17;18;19	2026-09-23	pgcolumnar_index_build_range_scan errors, so CREATE INDEX fails
 index_fetch_penalty_crossover	index_fetch_penalty_crossover	a 50000-row correlated range uses the custom scan, not a fetching index	15;16;17;18;19	never	-
 index_fetch_penalty_crossover	index_fetch_penalty_crossover	a selective point lookup still uses the index	15;16;17;18;19	never	-
 index_fetch_penalty_crossover	index_fetch_penalty_crossover	both paths return the same aggregate at 50000	15;16;17;18;19	never	-

test/check_ledger_budget.txt

@@ -594,4 +594,38 @@ suites_not_covered 249
 # YOU KNOW WHAT IS THERE.
 #
 # suites_not_covered does NOT move: catalog_plan_index is already covered.
-checks_never_observed_red 1521
+#
+# 1521 -> 1512 for #1236's second batch: nine index_am_support arms attacked, after
+# this branch was rebased onto a main carrying #1241. RE-DERIVED BY COUNTING on
+# the composed tree -- the branch carried 1510 against the previous main and that
+# number does not survive, because it described a tree that no longer exists:
+#
+#     awk -F'\t' '$5=="never"' test/check_ledger.tsv | wc -l   ->  1512
+#     1606 rows total, 94 not `never`
+#
+# THE SAME SHAPE AS THE FIRST BATCH. index_am_support's only dated rows were two
+# premises about docs/features.md, reddened by a DOCS mutation, which cannot
+# reach one arm about what an index does. Eight behaviour arms sat `never` next
+# to them, unattacked rather than vacuous.
+#
+# TWO MUTATIONS WITH DISJOINT PREDICTED SETS, both confirmed exactly:
+#
+#   M-1  the build drops two rows in three, so the index misses entries
+#        predicted the four "answers its operator" arms and NOT the build arms
+#        observed  exactly those four; CREATE INDEX still succeeds, so every
+#                  "builds an index" arm stayed green
+#
+#   M-2  the build errors, so CREATE INDEX fails
+#        predicted the four "builds an index" arms
+#        observed  those four, plus the GiST overlap premise, which needs an
+#                  index that can no longer be built
+#
+# AND M-2 SHOWS A WEAKNESS IN THE FOUR ANSWER ARMS, which is worth recording
+# where the rows are. With NO index in existence they all stayed GREEN: the arm
+# compares a count taken with `enable_seqscan = off` against one taken with the
+# custom scan off, and `enable_seqscan` is a preference rather than a
+# prohibition, so with no index both sides fall back to the same scan and agree.
+# The arm is named "answers its operator with the same rows the scan returns"
+# and can be satisfied without an index scan happening. Reported on #1236 rather
+# than repaired here: this change records evidence and alters no test.
+checks_never_observed_red 1512