MDEV-18266 Changing an index comment unnecessarily rebuilds index

ALTER_CHANGE_INDEX_COMMENT: new handler flag added

Compare_keys::EqualButComment: new outcome of compare_keys_but_name()

mysql-test/suite/innodb/r/instant_alter.result

@@ -2533,3 +2533,17 @@ WHERE variable_name = 'innodb_instant_alter_column';
 instants
 181
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;
+#
+# MDEV-18266: Changing an index comment unnecessarily rebuilds index
+#
+CREATE TABLE t1(a INT, b INT) ENGINE=INNODB;
+CREATE INDEX i1 ON t1(a) COMMENT 'comment1';
+ALTER TABLE t1 DROP INDEX i1, ADD INDEX i1(a) COMMENT 'comment2', ALGORITHM=INSTANT;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL,
+  `b` int(11) DEFAULT NULL,
+  KEY `i1` (`a`) COMMENT 'comment2'
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+DROP TABLE t1;

mysql-test/suite/innodb/t/instant_alter.test

@@ -752,3 +752,13 @@ SELECT variable_value-@old_instant instants
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;
+
+--echo #
+--echo # MDEV-18266: Changing an index comment unnecessarily rebuilds index
+--echo #
+
+CREATE TABLE t1(a INT, b INT) ENGINE=INNODB;
+CREATE INDEX i1 ON t1(a) COMMENT 'comment1';
+ALTER TABLE t1 DROP INDEX i1, ADD INDEX i1(a) COMMENT 'comment2', ALGORITHM=INSTANT;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;

sql/handler.h

@@ -643,6 +643,7 @@ typedef ulonglong alter_table_operations;
 #define ALTER_ADD_FOREIGN_KEY       (1ULL << 21)
 // Set for DROP FOREIGN KEY
 #define ALTER_DROP_FOREIGN_KEY      (1ULL << 22)
+#define ALTER_CHANGE_INDEX_COMMENT  (1ULL << 23)
 // Set for ADD [COLUMN] FIRST | AFTER
 #define ALTER_COLUMN_ORDER          (1ULL << 25)
 #define ALTER_ADD_CHECK_CONSTRAINT  (1ULL << 27)

sql/lex_string.h

@@ -37,6 +37,10 @@ static inline bool cmp(const LEX_CSTRING *a, const LEX_CSTRING *b)
   return (a->length != b->length ||
           memcmp(a->str, b->str, a->length));
 }
+static inline bool cmp(const LEX_CSTRING a, const LEX_CSTRING b)
+{
+  return a.length != b.length || memcmp(a.str, b.str, a.length);
+}
 
 /*
   Compare if two LEX_CSTRING are equal. Assumption is that

sql/sql_table.cc

@@ -6521,10 +6521,11 @@ static int compare_uint(const uint *s, const uint *t)
   return (*s < *t) ? -1 : ((*s > *t) ? 1 : 0);
 }
 
-enum class Compare_keys
+enum class Compare_keys : uint32_t
 {
   Equal,
   EqualButKeyPartLength,
+  EqualButComment,
   NotEqual
 };
 
@@ -6608,11 +6609,12 @@ Compare_keys compare_keys_but_name(const KEY *table_key, const KEY *new_key,
     return Compare_keys::NotEqual;
 
   /* Check that key comment is not changed. */
-  if (table_key->comment.length != new_key->comment.length ||
-      (table_key->comment.length &&
-       memcmp(table_key->comment.str, new_key->comment.str,
-              table_key->comment.length) != 0))
-    return Compare_keys::NotEqual;
+  if (cmp(table_key->comment, new_key->comment) != 0)
+  {
+    if (result != Compare_keys::Equal)
+      return Compare_keys::NotEqual;
+    result= Compare_keys::EqualButComment;
+  }
 
   return result;
 }
@@ -7002,6 +7004,9 @@ static bool fill_alter_inplace_info(THD *thd, TABLE *table, bool varchar,
     case Compare_keys::EqualButKeyPartLength:
       ha_alter_info->handler_flags|= ALTER_COLUMN_INDEX_LENGTH;
       continue;
+    case Compare_keys::EqualButComment:
+      ha_alter_info->handler_flags|= ALTER_CHANGE_INDEX_COMMENT;
+      continue;
     case Compare_keys::NotEqual:
       break;
     }

storage/innobase/handler/handler0alter.cc

@@ -108,7 +108,8 @@ static const alter_table_operations INNOBASE_INPLACE_IGNORE
 	| ALTER_VIRTUAL_GCOL_EXPR
 	| ALTER_DROP_CHECK_CONSTRAINT
 	| ALTER_RENAME
-	| ALTER_COLUMN_INDEX_LENGTH;
+	| ALTER_COLUMN_INDEX_LENGTH
+	| ALTER_CHANGE_INDEX_COMMENT;
 
 /** Operations on foreign key definitions (changing the schema only) */
 static const alter_table_operations INNOBASE_FOREIGN_OPERATIONS