MySql JOIN performance go down with group by - mysql

Question

I have these tables:
table "f" (26000 record)
+------------------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------+------------------+------+-----+---------+-------+
| idFascicolo | int(11) | NO | PRI | | |
| oggetto | varchar | NO |index| | |
+------------------+------------------+------+-----+---------+-------+
table "r" (22000 record)
+------------------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------+------------------+------+-----+---------+-------+
| idRichiedente | int(11) | NO | PRI | | |
| name | varchar | NO |index| | |
+------------------+------------------+------+-----+---------+-------+
table "fr" (32000 record)
+------------------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------+------------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | | |
| idFascicolo | int(11) | NO |index| | FK |
| idRichiedente | int(11) | NO |index| | FK |
+------------------+------------------+------+-----+---------+-------+
this is my select:
SELECT
f.idFascicolo,
f.oggetto,
r.richiedente
FROM fr
JOIN f ON (f.idFascicolo=fr.idFascicolo)
JOIN r ON (r.idRichiedente=fr.idRichiedente)
WHERE r.name LIKE '%string%'
in the result, I would like to see only 1 row per f.idFascicolo (I should have "Rossi Mario" and "Rossi Marco" for the same f.idFascicolo) , the my new select is:
SELECT
f.idFascicolo,
f.oggetto,
r.richiedente
FROM fr
JOIN f ON (f.idFascicolo=fr.idFascicolo)
JOIN r ON (r.idRichiedente=fr.idRichiedente)
WHERE r.name LIKE '%string%'
GROUP BY f.idFascicolo
here, the performance read from PhpMyAdmin:
0.0057 seconds: .. WHERE r.name LIKE '%string%'
0.0527 seconds: .. WHERE r.name LIKE '%string%' GROUP BY f.idFascicolo
0.0036 seconds: .. WHERE r.name LIKE 'string%' GROUP BY f.idFascicolo
I don't understand if the problem of the slow query is GROUP BY or LIKE '%string%'(i need '%string%' .. I can't find an equivalent solution with fulltext index and MATCH .. AGAINST)
This is the explain:
+------+-------------+-------+------+-------------------------+---------------+---------+----------------------+-----------+---------------------------------------------+
| id | select type | table | type | possible keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+------+-------------------------+---------------+---------+----------------------+-----------+---------------------------------------------+
| 1 | simple | r | ALL | PRIMARY | NULL | NULL | NULL | 20925 |Using where; Using temporary; Using filesort |
+------+-------------+-------+------+-------------------------+---------------+---------+----------------------+-----------+---------------------------------------------+
| 1 | simple | fr | ref |idFascicolo,idRichiedente| idRichiedente | 4 | db.r.idRichiedente | 1 | |
+------+-------------+-------+------+-------------------------+---------------+---------+----------------------+-----------+---------------------------------------------+
| 1 | simple | f |eq_ref|PRIMARY | PRIMARY | 4 | db.fr.idFascicolo | 1 | |
+------+-------------+-------+------+-------------------------+---------------+---------+----------------------+-----------+---------------------------------------------+

Answer 1

You have two potential performance issues. First is the GROUP BY. This requires sorting the data, so it has to read all the data and do a lot of work.
The second is the LIKE. There is a fundamental difference between:
WHERE r.name LIKE '%string%'
and
WHERE r.name LIKE 'string%'
The second can use an index on r(name), because the like pattern does not start with a pattern.
I am not sure what your actual question is. I don't recommend doing using GROUP BY the way you are using it -- because you have unaggregated columns in the SELECT.