create table t1 (c1 varchar(30),c2 varchar(150),unique index t1_idx (c2,c1));

insert into t1 values ('a','a'),('b','b'),('c','c');

delimiter |;

create procedure p1()

begin

  declare x varchar(30);

  select max(c1) into @x from t1 where c2='a';

  select found_rows();

  select @x;

end;

|

delimiter ;|

call p1();

drop index t1_idx on t1;

call p1();

drop procedure p1;

drop table t1;

MariaDB [test]> call p1();

+--------------+

| found_rows() |

+--------------+

|            0 |

+--------------+

1 row in set (0.001 sec)

+------+

| @x   |

+------+

| a    |

+------+

1 row in set (0.001 sec)

Query OK, 1 row affected (0.001 sec)

MariaDB [test]> drop index t1_idx on t1;

Query OK, 0 rows affected (0.020 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> call p1();

+--------------+

| found_rows() |

+--------------+

|            1 |

+--------------+

1 row in set (0.000 sec)

+------+

| @x   |

+------+

| a    |

+------+

1 row in set (0.000 sec)

Query OK, 1 row affected (0.000 sec)