1 files changed, 31 insertions, 0 deletions
diff --git a/mysql-test/suite/innodb/t/instant_alter_charset.test b/mysql-test/suite/innodb/t/instant_alter_charset.test
index eeb6fa5e241..5be4f7a5b83 100644
--- a/mysql-test/suite/innodb/t/instant_alter_charset.test
+++ b/mysql-test/suite/innodb/t/instant_alter_charset.test
@@ -857,3 +857,34 @@ create table t2 (input_id int primary key, id int not null,
 alter table t1 change id id2 int;
 drop table t2;
 drop table t1;
+
+
+--echo #
+--echo # MDEV-25951 MariaDB crash after ALTER TABLE convert to utf8mb4
+--echo #
+
+CREATE TABLE t1 (id INT PRIMARY KEY, a VARCHAR(32), KEY (a(7))) ENGINE=INNODB DEFAULT CHARSET=UTF8;
+INSERT INTO t1 VALUES (1, 'a1'), (2, 'a1');
+
+--error ER_DUP_ENTRY
+ALTER TABLE t1
+  CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_520_ci,
+  ADD UNIQUE INDEX test_key (a);
+
+ALTER TABLE t1 CONVERT TO CHARACTER SET UTF8MB4 COLLATE UTF8MB4_UNICODE_520_CI;
+CHECK TABLE t1;
+SELECT * FROM t1;
+DROP TABLE t1;
+
+CREATE TABLE t1 (id INT PRIMARY KEY, a CHAR(32), KEY (a(7))) ENGINE=INNODB DEFAULT CHARSET=UTF8;
+INSERT INTO t1 VALUES (1, 'a1'), (2, 'a1');
+
+--error ER_DUP_ENTRY
+ALTER TABLE t1
+  CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_520_ci,
+  ADD UNIQUE INDEX test_key (a);
+
+ALTER TABLE t1 CONVERT TO CHARACTER SET UTF8MB4 COLLATE UTF8MB4_UNICODE_520_CI;
+CHECK TABLE t1;
+SELECT * FROM t1;
+DROP TABLE t1;