MDEV-20726 InnoDB: Assertion failure in file data0type.cc line 67bb-10.4-MDEV-20726-alter-utf8

Do not rebuild index when it's key part converted from utf8mb3 to utf8mb4
but key part stays the same.

dict_index_add_to_cache(): assert that prefix_len is divided by mbmaxlen

ha_innobase::compare_key_parts(): compare key part lenght in symbols instead
of bytes.

2 files changed, 26 insertions, 0 deletions

diff --git a/mysql-test/suite/innodb/r/instant_alter_charset.result b/mysql-test/suite/innodb/r/instant_alter_charset.result
index f8c21978327..cbb49819bfe 100644
--- a/mysql-test/suite/innodb/r/instant_alter_charset.result
+++ b/mysql-test/suite/innodb/r/instant_alter_charset.result
@@ -1918,3 +1918,14 @@ check table t;
 Table	Op	Msg_type	Msg_text
 test.t	check	status	OK
 drop table t;
+#
+# MDEV-20726: InnoDB: Assertion failure in file data0type.cc line 67
+#
+CREATE TABLE t (
+id int(10) unsigned NOT NULL PRIMARY KEY,
+a text CHARSET utf8mb3,
+KEY a_idx(a(1))
+) ENGINE=InnoDB;
+INSERT INTO t VALUES (1, 'something in the air');
+ALTER TABLE t MODIFY a text CHARSET utf8mb4;
+DROP TABLE t;
diff --git a/mysql-test/suite/innodb/t/instant_alter_charset.test b/mysql-test/suite/innodb/t/instant_alter_charset.test
index 82ae1040f57..270db2d6462 100644
--- a/mysql-test/suite/innodb/t/instant_alter_charset.test
+++ b/mysql-test/suite/innodb/t/instant_alter_charset.test
@@ -715,3 +715,18 @@ alter table t modify c varchar(10) collate latin1_general_cs, algorithm=instant;
 check table t;
 
 drop table t;
+
+--echo #
+--echo # MDEV-20726: InnoDB: Assertion failure in file data0type.cc line 67
+--echo #
+
+CREATE TABLE t (
+  id int(10) unsigned NOT NULL PRIMARY KEY,
+  a text CHARSET utf8mb3,
+  KEY a_idx(a(1))
+) ENGINE=InnoDB;
+
+INSERT INTO t VALUES (1, 'something in the air');
+ALTER TABLE t MODIFY a text CHARSET utf8mb4;
+
+DROP TABLE t;