From 5ba0ed98aafc304f6a6be3c1015c2e6f85135c82 Mon Sep 17 00:00:00 2001
From: Harshil Mital <harshil.mital20@gmail.com>
Date: Fri, 23 Oct 2026 08:47:27 +0000
Subject: [PATCH] fix: SortPreservingMergeExec drops input orderings when
 merging partitions

Merging multiple partitions only preserves the merge key ordering. Other
input orderings hold within each partition only, so keeping them let the
planner remove a sort that was still needed and return wrong results.
---
 .../src/sorts/sort_preserving_merge.rs        |  5 ++
 datafusion/sqllogictest/test_files/order.slt  | 57 +++++++++++++++++++
 2 files changed, 62 insertions(+)

diff --git a/datafusion/physical-plan/src/sorts/sort_preserving_merge.rs b/datafusion/physical-plan/src/sorts/sort_preserving_merge.rs
index e865fef98fee7..febc494bfdb7e 100644
--- a/datafusion/physical-plan/src/sorts/sort_preserving_merge.rs
+++ b/datafusion/physical-plan/src/sorts/sort_preserving_merge.rs
@@ -172,6 +172,11 @@ impl SortPreservingMergeExec {
         };
 
         let mut eq_properties = input.equivalence_properties().clone();
+        // Merging multiple partitions only preserves the merge key ordering.
+        // Other input orderings hold only within each partition:
+        if input_partitions > 1 {
+            eq_properties.clear_orderings();
+        }
         eq_properties.clear_per_partition_constants();
         eq_properties.add_ordering(ordering);
         PlanProperties::new(
diff --git a/datafusion/sqllogictest/test_files/order.slt b/datafusion/sqllogictest/test_files/order.slt
index 3a2c7ccb844c9..0d48421a833b7 100644
--- a/datafusion/sqllogictest/test_files/order.slt
+++ b/datafusion/sqllogictest/test_files/order.slt
@@ -2276,3 +2276,60 @@ SELECT i, -i AS n FROM (
 -128 -128
 -128 -128
 NULL NULL
+
+# SortPreservingMergeExec only guarantees its merge key ordering. Each file
+# below is sorted by both `a` and `b`, but merging them on `a` does not keep
+# them sorted by `b`, so the outer sort must stay.
+statement ok
+set datafusion.execution.target_partitions = 2;
+
+query I
+COPY (VALUES (1, 10), (2, 20))
+TO 'test_files/scratch/order/spm_orderings/1.csv'
+OPTIONS ('format.has_header' 'true');
+----
+2
+
+query I
+COPY (VALUES (1, 1), (3, 2))
+TO 'test_files/scratch/order/spm_orderings/2.csv'
+OPTIONS ('format.has_header' 'true');
+----
+2
+
+statement ok
+CREATE EXTERNAL TABLE spm_orderings (
+  a BIGINT,
+  b BIGINT
+)
+STORED AS CSV
+WITH ORDER (a ASC)
+WITH ORDER (b ASC)
+LOCATION 'test_files/scratch/order/spm_orderings/'
+OPTIONS ('format.has_header' 'true');
+
+query II
+SELECT * FROM (SELECT a, b FROM spm_orderings ORDER BY a LIMIT 10) ORDER BY b;
+----
+1 1
+3 2
+1 10
+2 20
+
+query TT
+EXPLAIN SELECT * FROM (SELECT a, b FROM spm_orderings ORDER BY a LIMIT 10) ORDER BY b;
+----
+logical_plan
+01)Sort: spm_orderings.b ASC NULLS LAST
+02)--Sort: spm_orderings.a ASC NULLS LAST, fetch=10
+03)----TableScan: spm_orderings projection=[a, b]
+physical_plan
+01)SortExec: expr=[b@1 ASC NULLS LAST], preserve_partitioning=[false]
+02)--SortPreservingMergeExec: [a@0 ASC NULLS LAST], fetch=10
+03)----DataSourceExec: file_groups={2 groups: [[WORKSPACE_ROOT/datafusion/sqllogictest/test_files/scratch/order/spm_orderings/1.csv], [WORKSPACE_ROOT/datafusion/sqllogictest/test_files/scratch/order/spm_orderings/2.csv]]}, projection=[a, b], limit=10, output_orderings=[[a@0 ASC NULLS LAST], [b@1 ASC NULLS LAST]], file_type=csv, has_header=true
+
+statement ok
+DROP TABLE spm_orderings;
+
+statement ok
+set datafusion.execution.target_partitions = 4;