alamb commented on code in PR #14271:
URL: https://github.com/apache/datafusion/pull/14271#discussion_r1934326726
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datafusion/sqllogictest/test_files/aggregate.slt:
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@@ -6203,3 +6203,97 @@ physical_plan
14)--------------PlaceholderRowExec
15)------------ProjectionExec: expr=[1 as id, 2 as foo]
16)--------------PlaceholderRowExec
+
+
+# Set-Monotonic Aggregate functions can output results in order
+statement ok
+CREATE EXTERNAL TABLE aggregate_test_100_ordered (
+ c1 VARCHAR NOT NULL,
+ c2 TINYINT NOT NULL,
+ c3 SMALLINT NOT NULL,
+ c4 SMALLINT,
+ c5 INT,
+ c6 BIGINT NOT NULL,
+ c7 SMALLINT NOT NULL,
+ c8 INT NOT NULL,
+ c9 INT UNSIGNED NOT NULL,
+ c10 BIGINT UNSIGNED NOT NULL,
+ c11 FLOAT NOT NULL,
+ c12 DOUBLE NOT NULL,
+ c13 VARCHAR NOT NULL
+)
+STORED AS CSV
+LOCATION '../../testing/data/csv/aggregate_test_100.csv'
+WITH ORDER (c1)
+OPTIONS ('format.has_header' 'true');
+
+statement ok
+set datafusion.optimizer.prefer_existing_sort = true;
+
+query TT
+EXPLAIN SELECT c1, SUM(c9) as sum_c9 FROM aggregate_test_100_ordered GROUP BY
c1 ORDER BY c1, sum_c9;
+----
+logical_plan
+01)Sort: aggregate_test_100_ordered.c1 ASC NULLS LAST, sum_c9 ASC NULLS LAST
+02)--Projection: aggregate_test_100_ordered.c1,
sum(aggregate_test_100_ordered.c9) AS sum_c9
+03)----Aggregate: groupBy=[[aggregate_test_100_ordered.c1]],
aggr=[[sum(CAST(aggregate_test_100_ordered.c9 AS UInt64))]]
+04)------TableScan: aggregate_test_100_ordered projection=[c1, c9]
+physical_plan
+01)SortPreservingMergeExec: [c1@0 ASC NULLS LAST, sum_c9@1 ASC NULLS LAST]
+02)--ProjectionExec: expr=[c1@0 as c1, sum(aggregate_test_100_ordered.c9)@1 as
sum_c9]
+03)----AggregateExec: mode=FinalPartitioned, gby=[c1@0 as c1],
aggr=[sum(aggregate_test_100_ordered.c9)], ordering_mode=Sorted
Review Comment:
I don't understand this plan
It seem to say that if we know the data is sorted by `c1` (only) then after
a grouping that preserves the `c1` sort (`ordering_mode=Sorted`) we also know
the data is sorted by `c1, sum(c9)`
But that isn't always true, for example
| c1 | c9 |
|--------|--------|
| 1 | 100 |
| 1 | 200 |
| 2 | 10 |
| 2 | 20 |
| 2 | 30 |
The result is not ordered the same (it would need to be reordered)
| c1 | sum(c9) |
|--------|--------|
| 1 | 300 |
| 2 | 60 |
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