kosiew commented on code in PR #26150:
URL: https://github.com/apache/datafusion/pull/26150#discussion_r4228258449
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datafusion/sqllogictest/test_files/functional_dependencies.slt:
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@@ -358,6 +358,52 @@ drop table t_null;
statement ok
drop table t_probe;
+
+# 6. Only a column reference carries a dependency. `CAST(x AS INT)` is named
+# `t_cast.x`, like the PRIMARY KEY column, but it does not determine `y`:
+# 1.1 and 1.2 both cast to 1.
+statement ok
+CREATE TABLE t_cast (x DOUBLE, y VARCHAR, PRIMARY KEY (x)) AS VALUES (1.1,
'b'), (1.2, 'a'), (2.1, 'c');
+
+# 6.1 ORDER BY: `y` is the tie-breaker within `CAST(x AS INT) = 1` and is kept.
+query RT
+SELECT x, y FROM t_cast ORDER BY CAST(x AS INT), y;
+----
+1.2 a
+1.1 b
+2.1 c
+
+query TT
+EXPLAIN SELECT x, y FROM t_cast ORDER BY CAST(x AS INT), y;
+----
+logical_plan
+01)Sort: CAST(t_cast.x AS Int32) ASC NULLS LAST, t_cast.y ASC NULLS LAST
+02)--TableScan: t_cast projection=[x, y]
+
+# 6.2 GROUP BY: `y` is not selected but still splits the groups.
+query II rowsort
+SELECT CAST(x AS INT) k, count(*) n FROM t_cast GROUP BY CAST(x AS INT), y;
+----
+1 1
+1 1
+2 1
+
+# 6.3 GROUP BY output: the dependency `k -> y` does not exist, so `y` still
+# orders the rows within `k = 1`.
+query IT
+SELECT CAST(x AS INT) k, y FROM t_cast GROUP BY CAST(x AS INT), y ORDER BY k,
y;
+----
+1 a
+1 b
+2 c
+
+# 6.4 `y` is not determined by the GROUP BY expression, so it can't be
selected.
+query error DataFusion error: Error during planning: Column in SELECT must be
in GROUP BY or an aggregate function
+SELECT y, count(*) FROM t_cast GROUP BY CAST(x AS INT);
Review Comment:
Could we also add a regression test for `TRY_CAST` over a `UNIQUE NOT NULL`
key? For example, `('bad-a', 'b')` and `('bad-b', 'a')` both produce NULL when
cast to INT, so the test could verify that `ORDER BY` retains `y`, `GROUP BY`
preserves both groups, and selecting `y` when grouping only by the cast fails
planning. This is optional coverage since the new helper already handles
`TRY_CAST` conservatively.
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