Revert (MDEV-18888)2b06de8064660c5c, fix it in different way And add test case for MDEV-18953

mysql-test/main/disabled.def

@@ -22,5 +22,3 @@ partition_open_files_limit : open_files_limit check broken by MDEV-18360
 mysqlcheck               : special tables like proxy , host specific to a system are shown
 flush_read_lock          : special tables like proxy , host specific to a system are shown
 join_cache               : enable after MDEV-17752 is fixed
-ctype_utf8mb4_myisam     : enable after MDEV-18953 is fixed
-ctype_utf8mb4_innodb     : enable after MDEV-18953 is fixed

mysql-test/main/long_unique_bugs.result

@@ -219,3 +219,19 @@ data
 f
 o
 DROP TABLE t1;
+create table t1 (
+c char(10) character set utf8mb4,
+unique key a using hash (c(1))
+) engine=myisam;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `c` char(10) CHARACTER SET utf8mb4 DEFAULT NULL,
+  UNIQUE KEY `a` (`c`(1)) USING HASH
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+insert into t1 values ('б');
+insert into t1 values ('бб');
+ERROR 23000: Duplicate entry '' for key 'a'
+insert into t1 values ('ббб');
+ERROR 23000: Duplicate entry '' for key 'a'
+drop table t1;

mysql-test/main/long_unique_bugs.test

@@ -250,3 +250,18 @@ SELECT * FROM t1;
 DROP TABLE t1;
 --let $datadir= `select @@datadir`
 --remove_file $datadir/test/load.data
+
+#
+# MDEV-18953 Hash index on partial char field not working
+#
+create table t1 (
+ c char(10) character set utf8mb4,
+ unique key a using hash (c(1))
+) engine=myisam;
+show create table t1;
+insert into t1 values ('б');
+--error ER_DUP_ENTRY
+insert into t1 values ('бб');
+--error ER_DUP_ENTRY
+insert into t1 values ('ббб');
+drop table t1;

sql/table.cc

@@ -1204,6 +1204,8 @@ bool parse_vcol_defs(THD *thd, MEM_ROOT *mem_root, TABLE *table,
                        new (mem_root) Item_field(thd, keypart->field),
                        new (mem_root) Item_int(thd, length));
           list_item->fix_fields(thd, NULL);
+          keypart->field->vcol_info=
+            table->field[keypart->field->field_index]->vcol_info;
         }
         else
           list_item= new (mem_root) Item_field(thd, keypart->field);
@@ -3710,14 +3712,8 @@ enum open_frm_error open_table_from_share(THD *thd, TABLE_SHARE *share,
       for ( ; key_part < key_part_end; key_part++)
       {
         Field *field= key_part->field= outparam->field[key_part->fieldnr - 1];
-        /*
-         There's no need to create a prefix
-         Field for HA_KEY_ALG_LONG_HASH indexes, as they implement prefixing via
-         Iten_func_left anyway (see parse_vcol_defs())
-        */
         if (field->key_length() != key_part->length &&
-            !(field->flags & BLOB_FLAG) &&
-            key_info->algorithm != HA_KEY_ALG_LONG_HASH)
+            !(field->flags & BLOB_FLAG))
         {
           /*
             We are using only a prefix of the column as a key: