MDEV-18275 Live upgrade from 5.5 does not work: InnoDB stat tables are used before creation

let mysql_upgrade to work even when innodb_index_stats or
innodb_table_stats are missing.

(mysql_upgrade knows to ignore "unknown table" errors)

mysql-test/main/mysql_upgrade.result

@@ -624,3 +624,9 @@ connection default;
 drop user user3@localhost;
 drop table mysql.global_priv;
 rename table mysql.global_priv_bak to mysql.global_priv;
+drop view mysql.user_bak;
+drop table mysql.innodb_index_stats, mysql.innodb_table_stats;
+# mysql_upgrade --force --silent 2>&1
+drop table mysql.global_priv;
+rename table mysql.global_priv_bak to mysql.global_priv;
+# End of 10.4 tests

mysql-test/main/mysql_upgrade.test

@@ -269,3 +269,17 @@ connection default;
 drop user user3@localhost;
 drop table mysql.global_priv;
 rename table mysql.global_priv_bak to mysql.global_priv;
+
+#
+# MDEV-18275 Live upgrade from 5.5 does not work: InnoDB stat tables are used before creation
+#
+--source include/switch_to_mysql_user.inc
+drop view mysql.user_bak;
+drop table mysql.innodb_index_stats, mysql.innodb_table_stats;
+--echo # mysql_upgrade --force --silent 2>&1
+--exec $MYSQL_UPGRADE --force --silent 2>&1
+--remove_file $MYSQLD_DATADIR/mysql_upgrade_info
+drop table mysql.global_priv;
+rename table mysql.global_priv_bak to mysql.global_priv;
+
+--echo # End of 10.4 tests

scripts/mysql_system_tables_fix.sql

@@ -670,35 +670,19 @@ SHOW WARNINGS;
 # Convering the host name to lower case for existing users
 UPDATE user SET host=LOWER( host ) WHERE LOWER( host ) <> host;
 
-# fix bad data when upgrading from unfixed InnoDB (MDEV-13360)
-set @str="delete from innodb_index_stats where length(table_name) > 64";
-set @str=if(@have_innodb <> 0, @str, "set @dummy = 0");
-prepare stmt from @str;
-execute stmt;
-set @str=replace(@str, "innodb_index_stats", "innodb_table_stats");
-prepare stmt from @str;
-execute stmt;
-
-# update table_name and timestamp fields in the innodb stat tables
-set @str="alter table mysql.innodb_index_stats modify last_update timestamp not null default current_timestamp on update current_timestamp, modify table_name varchar(199)";
-set @str=if(@have_innodb <> 0, @str, "set @dummy = 0");
-prepare stmt from @str;
-execute stmt;
-
-set @str="alter table mysql.innodb_table_stats modify last_update timestamp not null default current_timestamp on update current_timestamp, modify table_name varchar(199)";
-set @str=if(@have_innodb <> 0, @str, "set @dummy = 0");
-prepare stmt from @str;
-execute stmt;
-
-set @str=replace(@str, "innodb_index_stats", "innodb_table_stats");
-prepare stmt from @str;
-execute stmt;
-
-SET @innodb_index_stats_fk= (select count(*) from information_schema.referential_constraints where constraint_schema='mysql' and table_name = 'innodb_index_stats' and referenced_table_name = 'innodb_table_stats' and constraint_name = 'innodb_index_stats_ibfk_1');
-SET @str=IF(@innodb_index_stats_fk > 0 and @have_innodb > 0, "ALTER TABLE mysql.innodb_index_stats DROP FOREIGN KEY `innodb_index_stats_ibfk_1`", "SET @dummy = 0");
-PREPARE stmt FROM @str;
-EXECUTE stmt;
-DROP PREPARE stmt; 
+DELIMITER //
+if @have_innodb then
+  # fix bad data when upgrading from unfixed InnoDB (MDEV-13360)
+  delete from innodb_index_stats where length(table_name) > 64;
+  delete from innodb_table_stats where length(table_name) > 64;
+
+  # update table_name and timestamp fields in the innodb stat tables
+  alter table innodb_index_stats modify last_update timestamp not null default current_timestamp on update current_timestamp, modify table_name varchar(199);
+  alter table innodb_table_stats modify last_update timestamp not null default current_timestamp on update current_timestamp, modify table_name varchar(199);
+
+  alter table innodb_index_stats drop foreign key if exists innodb_index_stats_ibfk_1;
+end if //
+DELIMITER ;
 
 # MDEV-4332 longer user names
 alter table user         modify User         char(80)  binary not null default '';