test: latest_reading holds the GROUP BY recipe to reading the table once (#1339)

test/check_ledger.tsv

@@ -2241,6 +2241,7 @@ latest_reading	latest_reading	premise: the document's index exists	15;16;17;18;1
 latest_reading	latest_reading	premise: the document's three recipe blocks were found	15;16;17;18;19	never	-
 latest_reading	latest_reading	premise: the indexed LATERAL form read the table at all	15;16;17;18;19	never	-
 latest_reading	latest_reading	premise: the table holds every row	15;16;17;18;19	never	-
+latest_reading	latest_reading	the GROUP BY form reads the table once	15;16;17;18;19	2026-10-16	the document's GROUP BY recipe as a per-device subquery over devices, which returns the same rows and reads the table once per device
 latest_reading	latest_reading	the GROUP BY form returns what DISTINCT ON returns	15;16;17;18;19	never	-
 latest_reading	latest_reading	the LATERAL form returns what DISTINCT ON returns	15;16;17;18;19	2026-10-12	the document's LATERAL recipe orders by ts ascending, so it returns each device's FIRST reading
 latest_reading	latest_reading	with the btree the LATERAL form reads under a tenth of the table	15;16;17;18;19	never	-

test/check_ledger_budget.txt

@@ -3534,4 +3534,15 @@ suites_not_covered 190
 #     never         3058 -  3 = 3055
 #     dated         1330 + 11 = 1341
 #     3055 + 1341 == 4396
+#
+# #1339, latest_reading's GROUP BY form is held to reading the table once (jdatcmd):
+# 3055 -> 3055. The document says the form reads the table once, and the arms compared
+# only its rows, so a per-device subquery returning the same rows passed. The new arm
+# arrives dated by exactly that mutation, red on each of 15-19 in both twins while the
+# rows arm stayed green. From five-major runs, 9 checks green on each of 15-19.
+#
+#     ledger rows   4396 +  1 = 4397
+#     never         3055 +  0 = 3055
+#     dated         1341 +  1 = 1342
+#     3055 + 1342 == 4397
 checks_never_observed_red 3055

test/latest_reading.sh

@@ -80,6 +80,13 @@ check_num "without the btree the LATERAL form reads the table once per device" \
 
 check "the GROUP BY form returns what DISTINCT ON returns" \
 	"$(q "$GROUP_SQL" | md5sum | cut -c1-12)" "$ORACLE"
+# The document says this form reads the table once (#1339): rows alone cannot tell it
+# from a per-device subquery, which returns the same rows and reads the table once
+# per device. Reading nothing is refused, as for the LATERAL form below.
+grp_blocks="$(blocks "$GROUP_SQL")"
+echo "-- table $pages pages; the GROUP BY form read $grp_blocks blocks"
+check_num "the GROUP BY form reads the table once" \
+	"$( [ -n "$grp_blocks" ] && [ "$grp_blocks" -ge 1 ] && at_least $((pages + 1 - grp_blocks)) 1 || echo "read ${grp_blocks:-nothing}" )" "1"
 
 q "$INDEX_SQL" >/dev/null
 check_num "premise: the document's index exists" \

test/pytest/TESTS.md

@@ -6600,7 +6600,7 @@ Removal proof: the document's `LATERAL` edited to `ORDER BY ts` (ascending) retu
 
 | test | what it holds |
 | --- | --- |
-| `test_the_latest_reading_recipe_does_what_the_document_says` | the document's three blocks are found; the table holds its rows; without the btree the `LATERAL` form reads the table about once per device; the `GROUP BY` form and, with the document's index, the `LATERAL` form return the rows `DISTINCT ON` returns; the indexed `LATERAL` reads at least one block and under a tenth of the table |
+| `test_the_latest_reading_recipe_does_what_the_document_says` | the document's three blocks are found; the table holds its rows; without the btree the `LATERAL` form reads the table about once per device; the `GROUP BY` form and, with the document's index, the `LATERAL` form return the rows `DISTINCT ON` returns; the `GROUP BY` form reads at least one block and no more than the table, as the document says (#1339); the indexed `LATERAL` reads at least one block and under a tenth of the table |
 
 ## 88. test_storage_row_repair.py: the alpha5 release notes' repair procedure works as written
 

test/pytest/test_latest_reading.py

@@ -6,10 +6,10 @@
 blocks in 5.6 s; a btree on (device, ts) plus LATERAL read 1,013 blocks in 45 ms;
 the same LATERAL without the btree read 8.7 million blocks in 12.6 minutes.
 
-Asserted here: each documented form returns the rows DISTINCT ON returns; with
-the btree the LATERAL form reads under a tenth of the table, and at least one
-block (a query that reads nothing would otherwise pass); without it, it reads the
-table about once per device.
+Asserted here: each documented form returns the rows DISTINCT ON returns; the
+GROUP BY form reads the table at most once; with the btree the LATERAL form reads
+under a tenth of the table, and at least one block (a query that reads nothing
+would otherwise pass); without it, it reads the table about once per device.
 
 ON A PRIVATE DATABASE (`pgc_own_db`): the document names its tables, `readings`
 and `devices`, so they cannot carry a per-test prefix. Independent of
@@ -95,6 +95,13 @@ def test_the_latest_reading_recipe_does_what_the_document_says(pgc_own_db, expec
 
     expect.text(str(_rows(conn, group_sql)), str(oracle),
                 "the GROUP BY form returns what DISTINCT ON returns")
+    # The document says this form reads the table once (#1339). Rows alone cannot tell
+    # it from a per-device subquery, which returns the same rows and reads the table
+    # once per device. Reading nothing also reads under the table, so a zero is refused.
+    grp = _blocks(conn, group_sql)
+    print(f"-- table {pages} pages; the GROUP BY form read {grp} blocks")
+    expect.at_least(pages + 1 - grp if grp else 0, 1,
+                    "the GROUP BY form reads the table once")
 
     with conn.cursor() as cur:
         cur.execute(index_sql)