MariaDB slow left join using modified index - mariadb

Question

I have an old MySQL query to find a free ID that always worked fine (0.0325 seconds). But now I have changed to MariaDB and it's extremely slow (over 30 seconds)
SELECT
START AS codice
FROM (
SELECT
codice +1 AS START
FROM `clfoco`
WHERE
SUBSTRING(codice,9) BETWEEN '0000001' AND '9999999' AND codice LIKE '1201__%'
) AS a
LEFT JOIN (
SELECT
codice
FROM `clfoco`
WHERE
SUBSTRING(codice,9) BETWEEN '0000001' AND '9999999' AND codice LIKE '1201__%'
) AS b ON a.start = b.codice
WHERE b.codice IS NULL
LIMIT 1
"codice" is the primary index
The old database was MySQL 5.1.73 and the new one is MariaDB 5.5.52.
I have replicated the table(import/export db and data), deleted and recreated the index but the query in the new database is always slow.
I have read answers where the changed index (codice+1) breaks the indexes and force MySQL to scan the whole table, but I dont think this is the case since in the old MySQL database the query is fast.
Any suggestion?
EDIT
TABLE
CREATE TABLE IF NOT EXISTS `clfoco` (
`codice` varchar(20) NOT NULL,
`id_anagra` int(9) NOT NULL,
`descri` varchar(100) DEFAULT NULL,
....
....
....
PRIMARY KEY (`codice`),
KEY `id_anagra` (`id_anagra`),
FULLTEXT KEY `descri` (`descri`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
Explain with Maria Db
| id|select_type|table |type |possible_keys|key |key_len|ref |rows |Extra
------------------------------------------------------------------------------
| 1 | SIMPLE |clfoco|range |PRIMARY |PRIMARY |62 |NULL|24549 |Using where; Using index
| 1 | SIMPLE |clfoco|index |PRIMARY |PRIMARY |62 |NULL|25182 |Using where; Using index; Using join buffer (flat, BNL join)
Explain with Mysql 5.1.73
| id|select_type|table |type |possible_keys|key |key_len|ref |rows |Extra
------------------------------------------------------------------------------
| 1 |PRIMARY |<derived2> |ALL |NULL |NULL |NULL |NULL|24661
| 1 |PRIMARY |<derived3> |ALL |NULL |NULL |NULL |NULL|24661 |Using where; Not exists
| 3 | DERIVED |clfoco |index |PRIMARY |PRIMARY |62 |NULL|25182 |Using where; Using index
| 2 | DERIVED |clfoco |index |PRIMARY |PRIMARY |62 |NULL|25182 |Using where; Using index;
NEW EDIT
After Rick James suggestion I have changed the query.
SELECT a.codice+1 AS START
FROM `clfoco` a
WHERE codice LIKE '1201__%'
AND(substring(b.codice,9) between '0000001' and '9999999')
AND NOT EXISTS (
SELECT *
FROM `clfoco`
WHERE codice LIKE '1201__%'
AND (substring(b.codice,9) between '0000001' and '9999999')
AND codice = a.codice+1 )
LIMIT 1;
The execution time on MariaDB is 0.0294 seconds
EXPLAIN EXTENDED:
id|select_type |table |type |possible_keys|key |key_len|ref |rows |filtered|Extra
1 |PRIMARY |a |range|PRIMARY |PRIMARY|62 |NULL|24549 |100.00 |Using where; Using index
2 |DEPENDENT SUBQUERY|clfoco|range|PRIMARY |PRIMARY|62 |NULL|24549 |100.00 |Range checked for each record (index map: 0x1)

Answer 1

SELECT a.codice+1 AS START
FROM `clfoco` a
WHERE codice LIKE '1201__%'
AND NOT EXISTS (
SELECT *
FROM `clfoco`
WHERE codice LIKE '1201__%'
AND codice = a.codice+1 )
LIMIT 1;
Notes:
EXISTS may optimize better. (But your versions are quite old.)
SUBSTRING gets in the way of index usage.
The SUBSTRING check seems to be useless.
codice needs to be VARCHAR, else LIKE will be problematical.
Please provide SHOW CREATE TABLE and EXPLAIN SELECT ....

Answer 2

The subselects seem a little obfuscatory, which is always dangerous, because even if it doesn't confuse the optimizer now, it may in a future version (as seems to have happened). I would have written this, with no subselects:
select a.codice+1 as codice
from clfoco a
left join clfoco b
on b.codice=a.codice+1
and (substring(b.codice,9) between '0000001' and '9999999')
and b.codice like '1201__%'
where b.codice is null
and (substring(a.codice,9) between '0000001' and '9999999')
and a.codice like '1201__%'
limit 1;
Does this confuse the optimizer less?
From the explain you added of the NOT EXISTS version, it looks like it is using the like '1201__%' for indexing the dependent subquery when it shouldn't. Try this:
select a.codice+1 as codice
from clfoco a
left join clfoco b on b.codice=a.codice+1
where b.codice is null
and (substring(a.codice,9) between '0000001' and '9999999')
and a.codice like '1201__%'
and (substring(a.codice+1,9) between '0000001' and '9999999')
and a.codice+1 like '1201__%'
limit 1;