From 18dbeae1b86f1b6c0c29acd02e7e0dfbca91181c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marko=20M=C3=A4kel=C3=A4?= <marko.makela@mariadb.com>
Date: Wed, 15 Jan 2025 07:31:33 +0200
Subject: [PATCH] MDEV-35849: index records in a wrong order
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page_cur_dtuple_cmp(): Take DESC index fields into account also when
comparing a NULL value in a ROW_FORMAT≠REDUNDANT record.

Thanks to Elena Stepanova for finding this bug.
---
 mysql-test/suite/innodb/r/innodb-index.result | 10 ++++++++++
 mysql-test/suite/innodb/t/innodb-index.test   | 11 +++++++++++
 storage/innobase/page/page0cur.cc             |  2 +-
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/mysql-test/suite/innodb/r/innodb-index.result b/mysql-test/suite/innodb/r/innodb-index.result
index 98f711b314a..27733c5165d 100644
--- a/mysql-test/suite/innodb/r/innodb-index.result
+++ b/mysql-test/suite/innodb/r/innodb-index.result
@@ -1996,4 +1996,14 @@ a
 c
 DROP TABLE t1;
 # End of 10.8 tests
+#
+# MDEV-35849: index records in a wrong order
+#
+CREATE TABLE t (pk INT PRIMARY KEY, a VARCHAR(1), KEY (a DESC)) ENGINE=InnoDB;
+INSERT INTO t VALUES (1,NULL),(2,'x');
+CHECK TABLE t;
+Table	Op	Msg_type	Msg_text
+test.t	check	status	OK
+DROP TABLE t;
+# End of 11.8 tests
 ALTER DATABASE test CHARACTER SET utf8mb4 COLLATE utf8mb4_uca1400_ai_ci;
diff --git a/mysql-test/suite/innodb/t/innodb-index.test b/mysql-test/suite/innodb/t/innodb-index.test
index eb5a2c33738..c70352a49f3 100644
--- a/mysql-test/suite/innodb/t/innodb-index.test
+++ b/mysql-test/suite/innodb/t/innodb-index.test
@@ -1229,6 +1229,17 @@ DROP TABLE t1;
 
 --echo # End of 10.8 tests
 
+--echo #
+--echo # MDEV-35849: index records in a wrong order
+--echo #
+
+CREATE TABLE t (pk INT PRIMARY KEY, a VARCHAR(1), KEY (a DESC)) ENGINE=InnoDB;
+INSERT INTO t VALUES (1,NULL),(2,'x');
+CHECK TABLE t;
+DROP TABLE t;
+
+--echo # End of 11.8 tests
+
 --disable_query_log
 
 call mtr.add_suppression("InnoDB: Tablespace .* was not found at .*t[12].ibd.");
diff --git a/storage/innobase/page/page0cur.cc b/storage/innobase/page/page0cur.cc
index 00e29caeeb7..d1d11b50de8 100644
--- a/storage/innobase/page/page0cur.cc
+++ b/storage/innobase/page/page0cur.cc
@@ -606,7 +606,7 @@ static int page_cur_dtuple_cmp(const dtuple_t &dtuple, const rec_t *rec,
         {
           if (i < cur_field || dtuple.fields[i].len == UNIV_SQL_NULL)
             continue;
-          ret= 1;
+          ret= field->descending ? -1 : 1;
           break;
         }
       }