chore: add ordering tests for array_union (#4688)

Author: comphead

spark/src/test/resources/sql-tests/expressions/array/array_union.sql

@@ -84,6 +84,32 @@ SELECT array_union(array(1), array(1)) FROM test_union_dups
 query
 SELECT array_union(array(1), array(2)) FROM test_union_dups
 
+-- left-first-then-new-right ordering verification (issue: array_union ordering audit)
+-- Spark contract: array_union emits left-array elements first (deduped, in
+-- first-occurrence order), followed by right-array elements that are not
+-- already present. The cases below intentionally scramble element order so
+-- any deviation from "left order, then new right elements" surfaces in the
+-- result -- e.g. if the engine emitted in right-side order, sorted, or by
+-- some other strategy, these expectations would diverge from Spark.
+statement
+CREATE TABLE test_union_ordering(a array<int>, b array<int>) USING parquet
+
+statement
+INSERT INTO test_union_ordering VALUES (array(5, 4, 3, 2, 1), array(1, 2, 3, 4, 5, 6, 7)), (array(3, 1, 4, 1, 5, 9, 2, 6), array(7, 5, 3, 8)), (array(10, 20, 30), array(30, 25, 20, 15, 10)), (array(1, 2, 3), array(0, -1, -2)), (array(7, 7, 7, 8, 8), array(9, 8, 7)), (array(2, 1, 2, 1, 2), array(3, 2, 1)), (array(100), array(99, 100, 98, 100, 97)), (array(2, 1), array(1, 2)), (array(1, 2), array(2, 1))
+
+query
+SELECT a, b, array_union(a, b) FROM test_union_ordering
+
+-- left longer than right (the array_intersect ordering caveat does not apply
+-- here because array_union always iterates left then right; verify explicitly)
+query
+SELECT array_union(array(5, 4, 3, 2, 1, 0), array(2, 1)) FROM test_union_ordering
+
+-- right longer than left (the inverse of the above; would expose any
+-- "longest-side-first" probing strategy)
+query
+SELECT array_union(array(2, 1), array(5, 4, 3, 2, 1, 0)) FROM test_union_ordering
+
 -- string arrays
 statement
 CREATE TABLE test_union_str(a array<string>, b array<string>) USING parquet