test(sort-pushdown): cover DESC multi-row-group files with a shared min

Add Test 8b: a DESC scan over a file whose row groups share the same min
value (`[10,8,8,8]` → row groups `[10,8]` and `[8,8]`), listed out of order
on disk so the Inexact→Exact upgrade fires and SortExec is eliminated.

Without the fix this returns `8,8,10,8,...` (id=10 in third position): the
opener reorders row groups ASC-by-min then reverses, mis-ordering the two
min=8 row groups, and the eliminated SortExec no longer masks it. The test
asserts both the clean plan (no runtime reorder hints) and the correct
DESC result.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adriangb

datafusion/sqllogictest/test_files/sort_pushdown.slt

@@ -1486,6 +1486,78 @@ SELECT * FROM desc_reversed_parquet ORDER BY id DESC;
 2 200
 1 100
 
+# Test 8b: DESC with multiple row groups per file sharing a min value.
+# Regression test for the Inexact→Exact upgrade: when SortExec is eliminated
+# the files must be read in natural order. The opener's runtime row-group
+# reorder (sort ASC-by-min then reverse) mis-orders two row groups in one file
+# that share the same min — so the upgrade must NOT leave those hints active.
+#
+# File b_high is DESC-sorted [10,8,8,8] written with 2 rows per row group:
+#   RG0 = [10, 8] (min 8, max 10)
+#   RG1 = [ 8, 8] (min 8, max 8)
+# Both row groups have min=8. Naively reordering RGs ASC-by-min then reversing
+# yields [RG1, RG0] → 8,8,10,8 (wrong). Natural order [RG0, RG1] is correct.
+
+statement ok
+CREATE TABLE rg_desc_high(id INT, value INT) AS VALUES (10, 100), (8, 801), (8, 802), (8, 803);
+
+statement ok
+CREATE TABLE rg_desc_low(id INT, value INT) AS VALUES (3, 300), (2, 200), (1, 100);
+
+query I
+COPY (SELECT * FROM rg_desc_high ORDER BY id DESC)
+TO 'test_files/scratch/sort_pushdown/rg_desc/b_high.parquet'
+OPTIONS ('format.max_row_group_size' '2');
+----
+4
+
+query I
+COPY (SELECT * FROM rg_desc_low ORDER BY id DESC)
+TO 'test_files/scratch/sort_pushdown/rg_desc/a_low.parquet'
+OPTIONS ('format.max_row_group_size' '2');
+----
+3
+
+# Files named so filesystem order [a_low, b_high] is wrong for DESC → the
+# Inexact path fires, stats reorder makes file groups [b_high, a_low]
+# non-overlapping, and the upgrade eliminates SortExec.
+statement ok
+CREATE EXTERNAL TABLE rg_desc_parquet(id INT, value INT)
+STORED AS PARQUET
+LOCATION 'test_files/scratch/sort_pushdown/rg_desc/'
+WITH ORDER (id DESC);
+
+# SortExec eliminated, files reordered, NO sort_order_for_reorder /
+# reverse_row_groups (natural read is correct after the upgrade).
+query TT
+EXPLAIN SELECT id FROM rg_desc_parquet ORDER BY id DESC;
+----
+logical_plan
+01)Sort: rg_desc_parquet.id DESC NULLS FIRST
+02)--TableScan: rg_desc_parquet projection=[id]
+physical_plan DataSourceExec: file_groups={1 group: [[WORKSPACE_ROOT/datafusion/sqllogictest/test_files/scratch/sort_pushdown/rg_desc/b_high.parquet, WORKSPACE_ROOT/datafusion/sqllogictest/test_files/scratch/sort_pushdown/rg_desc/a_low.parquet]]}, projection=[id], output_ordering=[id@0 DESC], file_type=parquet
+
+# Results must be in DESC order — id=10 first.
+query I
+SELECT id FROM rg_desc_parquet ORDER BY id DESC;
+----
+10
+8
+8
+8
+3
+2
+1
+
+statement ok
+DROP TABLE rg_desc_parquet;
+
+statement ok
+DROP TABLE rg_desc_high;
+
+statement ok
+DROP TABLE rg_desc_low;
+
 # Test 9: Multi-column sort key validation
 # Files have (category, id) ordering. Files share a boundary value on category='B'
 # so column-level min/max statistics overlap on the primary key column.