MDEV-21245 InnoDB: Using a partial-field key prefix in search

ha_innobase::compare_key_parts(): If a full column index is
being replaced with a column prefix index, return Compare_keys::NotEqual.

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

@@ -90,3 +90,19 @@ worklog5743;
 col_1_text = REPEAT("a", 3500)	col_2_text  = REPEAT("o", 3500)
 1	1
 DROP TABLE worklog5743;
+#
+# MDEV-21245 InnoDB: Using a partial-field key prefix in search
+#
+CREATE TABLE t1 (a VARCHAR(255), KEY k(a)) DEFAULT CHARSET=utf8mb3
+ENGINE=InnoDB;
+INSERT INTO t1 set a='';
+alter table t1 change a a varchar(3000);
+affected rows: 0
+info: Records: 0  Duplicates: 0  Warnings: 1
+Warnings:
+Note	1071	Specified key was too long; max key length is 3072 bytes
+SELECT * FROM t1 WHERE a IN ('');
+a
+
+DROP TABLE t1;
+# End of 10.4 tests

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

@@ -93,3 +93,17 @@ SELECT col_1_text = REPEAT("a", 3500) , col_2_text  = REPEAT("o", 3500) FROM
 worklog5743;
 
 DROP TABLE worklog5743;
+
+--echo #
+--echo # MDEV-21245 InnoDB: Using a partial-field key prefix in search
+--echo #
+CREATE TABLE t1 (a VARCHAR(255), KEY k(a)) DEFAULT CHARSET=utf8mb3
+ENGINE=InnoDB;
+INSERT INTO t1 set a='';
+--enable_info
+alter table t1 change a a varchar(3000);
+--disable_info
+SELECT * FROM t1 WHERE a IN ('');
+DROP TABLE t1;
+
+--echo # End of 10.4 tests

storage/innobase/handler/ha_innodb.cc

@@ -21199,6 +21199,10 @@ Compare_keys ha_innobase::compare_key_parts(
     if (old_part.length >= new_part.length)
       return Compare_keys::NotEqual;
 
+    if (old_part.length == old_field.key_length() &&
+        new_part.length != new_field.key_length)
+      return Compare_keys::NotEqual;
+
     return Compare_keys::EqualButKeyPartLength;
   }