Please help me to know how to drop duplicate index with same name.
=== Error ===
alter table emp drop index fk_deptid;ERROR 1553 (HY000): Cannot drop index ‘fk_deptid’: needed in a foreign key constraint
=============

Below is scenario.

drop table if exists emp;
drop table if exists dept;
create table dept
(
deptid integer auto_increment primary key,
deptname varchar(20)
);

create table emp
(
empid integer auto_increment primary key,
empname varchar(20),
deptid integer,
key fk_deptid (deptid),
constraint fk_deptid foreign key(deptid) references dept(deptid)
);

* Find below output which shows indexes in the database. (Query for below output is at the end)

mysql> source list.sql
+————–+————+—————–+—————–+————-+———————–+————————+
| table_schema | table_name | constraint_name | constraint_type | column_list | referenced_table_name | referenced_column_name |
+————–+————+—————–+—————–+————-+———————–+————————+
| test | dept | PRIMARY | PRIMARY KEY | deptid | NULL | NULL |
| test | emp | fk_deptid | NON UNIQUE | deptid | NULL | NULL |
| test | emp | fk_deptid | FOREIGN KEY | deptid | dept | deptid |
| test | emp | PRIMARY | PRIMARY KEY | empid | NULL | NULL |
+————–+————+—————–+—————–+————-+———————–+————————+

alter table emp drop index fk_deptid;ERROR 1553 (HY000): Cannot drop index ‘fk_deptid’: needed in a foreign key constraint

+++Query++++
SELECT a.table_schema, a.table_name, a.constraint_name,
a.constraint_type,
convert(group_concat(DISTINCT b.column_name
ORDER BY b.ordinal_position SEPARATOR ‘, ‘), char)
as column_list,
b.referenced_table_name, b.referenced_column_name
FROM information_schema.table_constraints a
INNER JOIN information_schema.key_column_usage b
ON a.constraint_name = b.constraint_name AND
a.table_schema = b.table_schema AND
a.table_name = b.table_name
WHERE a.table_schema=’test’
GROUP BY a.table_schema, a.table_name, a.constraint_name,
a.constraint_type, b.referenced_table_name,
b.referenced_column_name
UNION
SELECT table_schema, table_name, index_name as constraint_name,
if(index_type=’FULLTEXT’, ‘FULLTEXT’, ‘NON UNIQUE’)
as constraint_type,
convert(group_concat(column_name
ORDER BY seq_in_index separator ‘, ‘), char) as column_list,
null as referenced_table_name, null as referenced_column_name
FROM information_schema.statistics
WHERE non_unique = 1
AND table_schema=’test’
GROUP BY table_schema, table_name, constraint_name, constraint_type,
referenced_table_name, referenced_column_name
ORDER BY table_schema, table_name, column_list

Thanks in advance.

Regards,
Amit.

index alpha (B,C)
index beta (A)

select A, B from x where A = x and B = x

Let’s say my Primary Key is: PRIMARY KEY(A,B)

I also need a standard BTREE index on B: KEY(B)

Now, would it be in my interest to change PRIMARY KEY(A,B) to PRIMARY KEY(B,A) and thus get the covering index on B and have no need for the additional KEY(B) ?

Thanks for your help and all the great information on this site.

This will catch indexes (A,B) and (A,C) as duplicate would not it ?
It would be simple if indexes would be single column only

SELECT * FROM information_schema.statistics WHERE table_schema = ‘?’ GROUP BY table_schema, table_name, column_name HAVING count(column_name) > 1;

It would be better to include ‘index_type’ in GROUP BY clause to more accurate answer.

In the above query bind the ‘database name’ parameter.

-Bog