Add SLT tests

Author: rkrishn7

datafusion/sqllogictest/test_files/order.slt

@@ -1554,3 +1554,65 @@ EXPLAIN SELECT * from ordered ORDER BY a;
 physical_plan
 01)SortExec: expr=[a@0 ASC NULLS LAST], preserve_partitioning=[false]
 02)--DataSourceExec: file_groups={1 group: [[WORKSPACE_ROOT/datafusion/sqllogictest/data/composite_order.csv]]}, projection=[a, b], output_ordering=[a@0 + b@1 ASC NULLS LAST], file_type=csv, has_header=true
+
+# Sort elimination through named_struct projections
+#
+# When data is sorted by a column and that column is wrapped into a struct
+# via named_struct, sorting by the struct field should NOT require a
+# separate SortExec because the optimizer can track that the struct field
+# preserves the original ordering.
+
+# Wrapping the ordered expression (a + b) into a struct field — sort eliminated
+# Reuses the `ordered` table above which has WITH ORDER (a + b).
+query TT
+EXPLAIN SELECT named_struct('sum', a + b) AS s FROM ordered ORDER BY s['sum'];
+----
+physical_plan
+01)ProjectionExec: expr=[named_struct(sum, a@0 + b@1) as s]
+02)--DataSourceExec: file_groups={1 group: [[WORKSPACE_ROOT/datafusion/sqllogictest/data/composite_order.csv]]}, projection=[a, b], output_ordering=[a@0 + b@1 ASC NULLS LAST], file_type=csv, has_header=true
+
+# Wrapping a non-ordered column into a struct — SortExec required
+# Reuses the `ordered` table above which has WITH ORDER (a + b).
+query TT
+EXPLAIN SELECT named_struct('a', a, 'b', b) AS s FROM ordered ORDER BY s['a'];
+----
+physical_plan
+01)SortExec: expr=[get_field(s@0, a) ASC NULLS LAST], preserve_partitioning=[false]
+02)--ProjectionExec: expr=[named_struct(a, a@0, b, b@1) as s]
+03)----DataSourceExec: file_groups={1 group: [[WORKSPACE_ROOT/datafusion/sqllogictest/data/composite_order.csv]]}, projection=[a, b], output_ordering=[a@0 + b@1 ASC NULLS LAST], file_type=csv, has_header=true
+
+# Simple column ordering tests using a table ordered by (a)
+statement ok
+CREATE EXTERNAL TABLE ordered_by_a (
+  a  BIGINT NOT NULL,
+  b  BIGINT NOT NULL
+)
+STORED AS CSV
+LOCATION 'data/composite_order.csv'
+OPTIONS ('format.has_header' 'true')
+WITH ORDER (a);
+
+# Single struct field matching source ordering -- sort eliminated
+query TT
+EXPLAIN SELECT named_struct('a', a, 'b', b) AS s FROM ordered_by_a ORDER BY s['a'];
+----
+physical_plan
+01)ProjectionExec: expr=[named_struct(a, a@0, b, b@1) as s]
+02)--DataSourceExec: file_groups={1 group: [[WORKSPACE_ROOT/datafusion/sqllogictest/data/composite_order.csv]]}, projection=[a, b], output_ordering=[a@0 ASC NULLS LAST], file_type=csv, has_header=true
+
+# Struct field not matching source ordering -- SortExec required
+query TT
+EXPLAIN SELECT named_struct('a', a, 'b', b) AS s FROM ordered_by_a ORDER BY s['b'];
+----
+physical_plan
+01)SortExec: expr=[get_field(s@0, b) ASC NULLS LAST], preserve_partitioning=[false]
+02)--ProjectionExec: expr=[named_struct(a, a@0, b, b@1) as s]
+03)----DataSourceExec: file_groups={1 group: [[WORKSPACE_ROOT/datafusion/sqllogictest/data/composite_order.csv]]}, projection=[a, b], output_ordering=[a@0 ASC NULLS LAST], file_type=csv, has_header=true
+
+# Mixed projection: top-level column alongside struct, order by struct field
+query TT
+EXPLAIN SELECT a, named_struct('a', a, 'b', b) AS s FROM ordered_by_a ORDER BY s['a'];
+----
+physical_plan
+01)ProjectionExec: expr=[a@0 as a, named_struct(a, a@0, b, b@1) as s]
+02)--DataSourceExec: file_groups={1 group: [[WORKSPACE_ROOT/datafusion/sqllogictest/data/composite_order.csv]]}, projection=[a, b], output_ordering=[a@0 ASC NULLS LAST], file_type=csv, has_header=true