MYSQL query : Lower method in sql disables my index - mysql

Question

how can have my query be able to not differienciate between my lower and upper case column query but without using the lower method of sql?
select * from
User
where
lower(lusername)= 'abdel'
i want to still be able to use my column name with any type of case (Abc, aBc, ABC,aBC..) but without using the lower function in my query sql because the lower disable my index that i put on my column name.
is there a way to do that ? thank you in advance

Answer 1

The default collation used in MySQL is case-insensitive. Therefore you don't need to use LOWER() if you are doing searching.
In other words, the following two queries should return the same results:
SELECT ... FROM mytable WHERE LOWER(name) = 'Abc';
SELECT ... FROM mytable WHERE name = 'Abc'
The latter query is able to optimize the query with an index, but the former can't.
Any expression or function call makes the search not "sargable" which means it can't use an index, because the optimizer can't know if that expression or function outputs have the same sort order as the index on the column.
Based on your comment below, I see you are using a collation utf8_bin, which is case-sensitive. This does improve performance a little bit over using a collation, so if performance is the only thing that's important in this case, I understand why you want to keep using that collation.
Here's a demo of using EXPLAIN to verify that it uses an index when you compare strings directly, but the binary collation does a case-sensitive search:
mysql> create table User (id serial primary key, lusername varchar(75) collate utf8_bin, created datetime, key(lusername));
mysql> explain select * from User where lusername = 'Abc';
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | User | NULL | ref | lusername | lusername | 228 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
But when we use a function like LOWER(), it can't use the index, and resorts to a table-scan:
mysql> explain select * from User where LOWER(lusername) = 'Abc';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | User | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
However, MySQL 5.7 has a feature to add a virtual column based on the expression, and index the virtual column:
mysql> alter table User add lower_lusername varchar(75) as (LOWER(lusername)), add key (lower_lusername);
mysql> explain select * from User where LOWER(lusername) = 'Abc';
+----+-------------+-------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | User | NULL | ref | lower_lusername | lower_lusername | 303 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
Alternatively, in MySQL 8.0, you can now get the index on the expression it without even adding a generated column:
mysql> alter table User add key ((LOWER(lusername)));
mysql> explain select * from User where LOWER(lusername) = 'Abc';
+----+-------------+-------+------------+------+------------------+------------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+------------------+------------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | User | NULL | ref | functional_index | functional_index | 228 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+------------------+------------------+---------+-------+------+----------+-------+