在mysql中使用和不使用索引执行查询

Question

I have to run the same queries with and without indexes, in mysql. I create indexes like this:

create index index_1 on table_1(column_name);
create index index_2 on table_2(column_name);

I execute this and I get the results that 0 rows has affected, both times. Is this ok?

Because when I execute the queries I have (after I created indexes), it takes me the same time as before (without indexes.)

The view of database:

I have multiple small queries about this database, eg

SELECT DISTINCT customers.customer_id, customers.customer_name
FROM customers
  INNER JOIN accounts ON customers.customer_id = accounts.customer_id
  INNER JOIN transactions ON transactions.account_id = accounts.account_id 
WHERE transactions.trn_date >= '2011/05/01'
  AND transactions.trn_date <= '2011/05/31'
ORDER BY customers.customer_id

Answer 1

You need these indexes for that query:

transactions: INDEX(trn_date)
accounts:     INDEX(account_id)
customers:    INDEX(customer_id)

In the last two cases, you probably already have that column as the PRIMARY KEY . If so, do not add a redundant INDEX .

Answer 2

I execute this and I get the results that 0 rows has affected, both times. Is this ok?

Yes, it is ok.
There are DDL operation which create new object and shouldn't output something.

Because when I execute the queries I have (after I created indexes) it takes me the same time as before (without indexes)

The queries haven't to use index. The internal optimizer make decision in depend of data distribution. For example, if column_name.table_1 has 50 unique values, then index is not be used.

More details ou can found in official documentation: 9.3.1 How MySQL Uses Indexes