From 9a0cbd31ce8576468981b14b066dea155cb922d9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marko=20M=C3=A4kel=C3=A4?= <marko.makela@mariadb.com>
Date: Mon, 4 Jul 2022 08:04:44 +0300
Subject: [PATCH] MDEV-26294 Duplicate entries in unique index not detected
 when changing collation

ha_innobase::check_if_supported_inplace_alter(): Refuse to change the
collation of a column that would become or remain indexed as part of
the ALTER TABLE operation.

In MariaDB Server 10.6, we will allow this type of operation;
that fix depends on MDEV-15250.
---
 .../innodb/r/change_column_collation.result   |  19 ++
 .../r/instant_alter_charset,redundant.rdiff   |  13 +-
 .../innodb/r/instant_alter_charset.result     | 178 ++++++++++++++++--
 .../innodb/t/change_column_collation.test     |  24 +++
 .../suite/innodb/t/instant_alter_charset.test |  23 +++
 storage/innobase/handler/handler0alter.cc     |  13 +-
 6 files changed, 249 insertions(+), 21 deletions(-)
 create mode 100644 mysql-test/suite/innodb/r/change_column_collation.result
 create mode 100644 mysql-test/suite/innodb/t/change_column_collation.test

diff --git a/mysql-test/suite/innodb/r/change_column_collation.result b/mysql-test/suite/innodb/r/change_column_collation.result
new file mode 100644
index 00000000000..fa1df1df830
--- /dev/null
+++ b/mysql-test/suite/innodb/r/change_column_collation.result
@@ -0,0 +1,19 @@
+#
+# MDEV-26294 Duplicate entries in unique index not detected when
+#	changing collation with INPLACE algorithm
+#
+SET NAMES utf8;
+CREATE TABLE t1 (
+id INT PRIMARY KEY,
+msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_swedish_ci UNIQUE
+) ENGINE=INNODB;
+INSERT INTO t1 VALUES (1, 'aaa');
+INSERT INTO t1 VALUES (2, 'ååå');
+ALTER TABLE t1
+MODIFY msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci,
+ALGORITHM=NOCOPY;
+ERROR 0A000: ALGORITHM=NOCOPY is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+ALTER TABLE t1 DROP INDEX msg,
+MODIFY msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci,
+ALGORITHM=NOCOPY;
+DROP TABLE t1;
diff --git a/mysql-test/suite/innodb/r/instant_alter_charset,redundant.rdiff b/mysql-test/suite/innodb/r/instant_alter_charset,redundant.rdiff
index 22b7e46ea9c..dd4caf125af 100644
--- a/mysql-test/suite/innodb/r/instant_alter_charset,redundant.rdiff
+++ b/mysql-test/suite/innodb/r/instant_alter_charset,redundant.rdiff
@@ -1,6 +1,4 @@
---- instant_alter_charset.result	2019-06-17 14:36:02.311515062 +0300
-+++ instant_alter_charset,redundant.result	2019-06-17 14:50:11.888705725 +0300
-@@ -279,7 +279,6 @@
+@@ -288,7 +288,6 @@
  alter table boundary_255
  modify a varchar(70) charset utf8mb4,
  algorithm=instant;
@@ -8,7 +6,7 @@
  drop table boundary_255;
  create table t (
  a char(10) collate utf8mb3_general_ci,
-@@ -297,32 +296,21 @@
+@@ -306,35 +305,21 @@
  repeat('a', 10), repeat('a', 10)
  );
  alter table t modify a char(10) collate utf8mb4_general_ci, algorithm=instant;
@@ -27,16 +25,19 @@
 -test.t	check	status	OK
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  alter table t modify aa char(10) collate utf8mb4_general_ci, algorithm=instant;
+-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 -check table t;
 -Table	Op	Msg_type	Msg_text
 -test.t	check	status	OK
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  alter table t modify bb char(70) collate utf8mb4_general_ci, algorithm=instant;
+-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 -check table t;
 -Table	Op	Msg_type	Msg_text
 -test.t	check	status	OK
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  alter table t modify cc char(100) collate utf8mb4_general_ci, algorithm=instant;
+-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 -check table t;
 -Table	Op	Msg_type	Msg_text
 -test.t	check	status	OK
@@ -44,8 +45,10 @@
  alter table t modify d char(10) collate utf8mb4_spanish_ci, algorithm=instant;
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  alter table t modify dd char(10) collate utf8mb4_spanish_ci, algorithm=instant;
--ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  select * from t;
  a	b	c	aa	bb	cc	d	dd
  aaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaa	aaaaaaaaaa
+
+mysqltest: Result length mismatch
diff --git a/mysql-test/suite/innodb/r/instant_alter_charset.result b/mysql-test/suite/innodb/r/instant_alter_charset.result
index 43b4b2453e8..8fcdca6fc93 100644
--- a/mysql-test/suite/innodb/r/instant_alter_charset.result
+++ b/mysql-test/suite/innodb/r/instant_alter_charset.result
@@ -199,6 +199,15 @@ a varchar(150) charset utf8mb3 unique key
 alter table key_part_bug
 modify a varchar(150) charset utf8mb4,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table key_part_bug
+modify a varchar(150) charset utf8mb4,
+algorithm=nocopy;
+ERROR 0A000: ALGORITHM=NOCOPY is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table key_part_bug
+drop index a,
+modify a varchar(150) charset utf8mb4,
+algorithm=nocopy;
 drop table key_part_bug;
 create table latin1_swedish_special_case (
 copy1 varchar(150) charset ascii collate ascii_general_ci,
@@ -309,20 +318,23 @@ check table t;
 Table	Op	Msg_type	Msg_text
 test.t	check	status	OK
 alter table t modify aa char(10) collate utf8mb4_general_ci, algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 check table t;
 Table	Op	Msg_type	Msg_text
 test.t	check	status	OK
 alter table t modify bb char(70) collate utf8mb4_general_ci, algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 check table t;
 Table	Op	Msg_type	Msg_text
 test.t	check	status	OK
 alter table t modify cc char(100) collate utf8mb4_general_ci, algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 check table t;
 Table	Op	Msg_type	Msg_text
 test.t	check	status	OK
 alter table t modify d char(10) collate utf8mb4_spanish_ci, algorithm=instant;
 alter table t modify dd char(10) collate utf8mb4_spanish_ci, algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 select * from t;
 a	b	c	aa	bb	cc	d	dd
 aaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaa	aaaaaaaaaa
@@ -378,6 +390,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_general_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_general_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_general_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -391,6 +407,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_bin,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_bin,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_bin,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -404,6 +424,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_unicode_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_unicode_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_unicode_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -417,6 +441,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_icelandic_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_icelandic_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_icelandic_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -430,6 +458,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_latvian_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_latvian_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_latvian_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -443,6 +475,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_romanian_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_romanian_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_romanian_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -456,6 +492,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_slovenian_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_slovenian_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_slovenian_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -469,6 +509,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_polish_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_polish_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_polish_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -482,6 +526,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_estonian_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_estonian_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_estonian_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -495,6 +543,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_spanish_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_spanish_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_spanish_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -508,6 +560,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_swedish_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_swedish_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_swedish_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -521,6 +577,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_turkish_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_turkish_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_turkish_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -534,6 +594,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_czech_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_czech_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_czech_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -547,6 +611,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_danish_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_danish_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_danish_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -560,6 +628,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_lithuanian_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_lithuanian_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_lithuanian_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -573,6 +645,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_slovak_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_slovak_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_slovak_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -586,6 +662,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_spanish2_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_spanish2_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_spanish2_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -599,6 +679,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_roman_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_roman_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_roman_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -612,6 +696,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_persian_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_persian_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_persian_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -625,6 +713,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_esperanto_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_esperanto_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_esperanto_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -638,6 +730,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_hungarian_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_hungarian_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_hungarian_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -651,6 +747,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_sinhala_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_sinhala_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_sinhala_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -664,6 +764,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_german2_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_german2_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_german2_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -677,6 +781,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_croatian_mysql561_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_croatian_mysql561_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_croatian_mysql561_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -690,6 +798,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_unicode_520_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_unicode_520_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_unicode_520_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -703,6 +815,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -716,6 +832,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_croatian_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_croatian_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_croatian_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -729,6 +849,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_myanmar_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_myanmar_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_myanmar_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -742,6 +866,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_thai_520_w2,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_thai_520_w2,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_thai_520_w2,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -755,6 +883,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_general_nopad_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_general_nopad_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_general_nopad_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -768,6 +900,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_nopad_bin,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_nopad_bin,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_nopad_bin,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -781,6 +917,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_unicode_nopad_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_unicode_nopad_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_unicode_nopad_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -794,6 +934,10 @@ alter table tmp
 change a a varchar(50) charset utf8mb4 collate utf8mb4_unicode_520_nopad_ci,
 modify b varchar(50) charset utf8mb4 collate utf8mb4_unicode_520_nopad_ci,
 algorithm=instant;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table tmp
+change a a varchar(50) charset utf8mb4 collate utf8mb4_unicode_520_nopad_ci,
+algorithm=instant;
 check table tmp;
 Table	Op	Msg_type	Msg_text
 test.tmp	check	status	OK
@@ -830,7 +974,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
 algorithm=instant;
@@ -847,7 +991,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
 algorithm=instant;
@@ -864,7 +1008,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
 algorithm=instant;
@@ -881,7 +1025,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset utf8mb4 collate utf8mb4_vietnamese_ci,
 algorithm=instant;
@@ -898,7 +1042,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset ascii collate ascii_bin,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset ascii collate ascii_bin,
 algorithm=instant;
@@ -915,7 +1059,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset utf8mb3 collate utf8mb3_lithuanian_ci,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset utf8mb3 collate utf8mb3_lithuanian_ci,
 algorithm=instant;
@@ -932,7 +1076,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset utf8mb4 collate utf8mb4_persian_ci,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset utf8mb4 collate utf8mb4_persian_ci,
 algorithm=instant;
@@ -949,7 +1093,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset utf8mb4 collate utf8mb4_german2_ci,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset utf8mb4 collate utf8mb4_german2_ci,
 algorithm=instant;
@@ -966,7 +1110,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset utf8mb3 collate utf8mb3_unicode_ci,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset utf8mb3 collate utf8mb3_unicode_ci,
 algorithm=instant;
@@ -983,7 +1127,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset latin1 collate latin1_general_ci,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset latin1 collate latin1_general_ci,
 algorithm=instant;
@@ -1000,7 +1144,7 @@ algorithm=instant;
 alter table tmp
 modify b varchar(50) charset utf16 collate utf16_german2_ci,
 algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table tmp
 modify c varchar(50) charset utf16 collate utf16_german2_ci,
 algorithm=instant;
@@ -1887,8 +2031,11 @@ insert into t values
 alter table t modify a char(10) collate latin1_general_cs, algorithm=inplace;
 ERROR 0A000: ALGORITHM=INPLACE is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
 alter table t modify b char(10) collate latin1_general_cs, algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table t modify b char(10) collate latin1_general_cs, algorithm=nocopy;
+ERROR 0A000: ALGORITHM=NOCOPY is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table t modify b char(10) collate latin1_general_cs,
+drop index b_key, algorithm=nocopy;
 check table t;
 Table	Op	Msg_type	Msg_text
 test.t	check	status	OK
@@ -1908,8 +2055,11 @@ insert into t values
 alter table t modify a varchar(10) collate latin1_general_cs, algorithm=inplace;
 ERROR 0A000: ALGORITHM=INPLACE is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
 alter table t modify b varchar(10) collate latin1_general_cs, algorithm=instant;
-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 alter table t modify b varchar(10) collate latin1_general_cs, algorithm=nocopy;
+ERROR 0A000: ALGORITHM=NOCOPY is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+alter table t modify b varchar(10) collate latin1_general_cs,
+drop index b_key, algorithm=nocopy;
 check table t;
 Table	Op	Msg_type	Msg_text
 test.t	check	status	OK
diff --git a/mysql-test/suite/innodb/t/change_column_collation.test b/mysql-test/suite/innodb/t/change_column_collation.test
new file mode 100644
index 00000000000..1ba519535bd
--- /dev/null
+++ b/mysql-test/suite/innodb/t/change_column_collation.test
@@ -0,0 +1,24 @@
+--source include/have_innodb.inc
+
+--echo #
+--echo # MDEV-26294 Duplicate entries in unique index not detected when
+--echo #	changing collation with INPLACE algorithm
+--echo #
+
+# Detect the duplicate entry after collation change of column
+
+SET NAMES utf8;
+CREATE TABLE t1 (
+  id INT PRIMARY KEY,
+  msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_swedish_ci UNIQUE
+) ENGINE=INNODB;
+INSERT INTO t1 VALUES (1, 'aaa');
+INSERT INTO t1 VALUES (2, 'ååå');
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+ALTER TABLE t1
+MODIFY msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci,
+ALGORITHM=NOCOPY;
+ALTER TABLE t1 DROP INDEX msg,
+MODIFY msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci,
+ALGORITHM=NOCOPY;
+DROP TABLE t1;
diff --git a/mysql-test/suite/innodb/t/instant_alter_charset.test b/mysql-test/suite/innodb/t/instant_alter_charset.test
index d64e30402ed..de60c24c0cd 100644
--- a/mysql-test/suite/innodb/t/instant_alter_charset.test
+++ b/mysql-test/suite/innodb/t/instant_alter_charset.test
@@ -227,9 +227,18 @@ create table key_part_bug (
   a varchar(150) charset utf8mb3 unique key
 ) engine=innodb;
 
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
 alter table key_part_bug
   modify a varchar(150) charset utf8mb4,
   algorithm=instant;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter table key_part_bug
+  modify a varchar(150) charset utf8mb4,
+  algorithm=nocopy;
+alter table key_part_bug
+  drop index a,
+  modify a varchar(150) charset utf8mb4,
+  algorithm=nocopy;
 
 drop table key_part_bug;
 
@@ -347,10 +356,13 @@ check table t;
 alter table t modify c char(100) collate utf8mb4_general_ci, algorithm=instant;
 check table t;
 
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
 alter table t modify aa char(10) collate utf8mb4_general_ci, algorithm=instant;
 check table t;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
 alter table t modify bb char(70) collate utf8mb4_general_ci, algorithm=instant;
 check table t;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
 alter table t modify cc char(100) collate utf8mb4_general_ci, algorithm=instant;
 check table t;
 
@@ -442,11 +454,16 @@ while ($counter <= $data_size) {
 
   insert into tmp values ('AAA', 'AAA'), ('bbb', 'bbb');
 
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
   eval alter table tmp
     change a a varchar(50) charset $to_charset collate $to_collate,
     modify b varchar(50) charset $to_charset collate $to_collate,
     algorithm=instant;
 
+  eval alter table tmp
+    change a a varchar(50) charset $to_charset collate $to_collate,
+    algorithm=instant;
+
   check table tmp;
 
   drop table tmp;
@@ -685,7 +702,10 @@ alter table t modify a char(10) collate latin1_general_cs, algorithm=inplace;
 
 --error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
 alter table t modify b char(10) collate latin1_general_cs, algorithm=instant;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
 alter table t modify b char(10) collate latin1_general_cs, algorithm=nocopy;
+alter table t modify b char(10) collate latin1_general_cs,
+drop index b_key, algorithm=nocopy;
 check table t;
 
 alter table t modify c char(10) collate latin1_general_cs, algorithm=instant;
@@ -708,7 +728,10 @@ alter table t modify a varchar(10) collate latin1_general_cs, algorithm=inplace;
 
 --error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
 alter table t modify b varchar(10) collate latin1_general_cs, algorithm=instant;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
 alter table t modify b varchar(10) collate latin1_general_cs, algorithm=nocopy;
+alter table t modify b varchar(10) collate latin1_general_cs,
+drop index b_key, algorithm=nocopy;
 check table t;
 
 alter table t modify c varchar(10) collate latin1_general_cs, algorithm=instant;
diff --git a/storage/innobase/handler/handler0alter.cc b/storage/innobase/handler/handler0alter.cc
index 480ca99c538..8c51f80f1d3 100644
--- a/storage/innobase/handler/handler0alter.cc
+++ b/storage/innobase/handler/handler0alter.cc
@@ -1,7 +1,7 @@
 /*****************************************************************************
 
 Copyright (c) 2005, 2019, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2021, MariaDB Corporation.
+Copyright (c) 2013, 2022, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -2310,7 +2310,16 @@ innodb_instant_alter_column_allowed_reason:
 
 			if (new_field->field) {
 				/* This is an existing column. */
-				continue;
+
+				if (new_field->field->charset()
+				    == key_part->field->charset()) {
+					continue;
+				}
+
+				ha_alter_info->unsupported_reason =
+					"Collation change on"
+					" an indexed column";
+				DBUG_RETURN(HA_ALTER_INPLACE_NOT_SUPPORTED);
 			}
 
 			/* This is an added column. */