Question

I have the talbe like that:

CREATE TABLE  UserTrans (
 `id` int(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `user_id` int(10) unsigned NOT NULL,
   `transaction_id` varchar(255) NOT NULL default '0',
  `source` varchar(100) NOT NULL,
   PRIMARY KEY (`id`),
   KEY `user_id` (`user_id`)
)

with innodb engine.

The transaction_id is var because sometimes it can be aphanumeric.

the id is the primary key.

so.. here is the thing, I have over 1M records. However, there is a query to check for duplicate transaciton_id on the specified source. So, here is my query:

SELECT * 
  FROM UserTrans 
 WHERE transaction_id = '212398043' 
   AND source = 'COMPANY_A';

this query getting very slow, like 2 seconds to run now. Should I index the transaction_id and the source?
e.g. KEY join_id (transaction_id, source)

What is the drawback if i do that?

Answer 1

Obviously the benefit is that it will improve the performance of certain queries.

The drawback is that it will take a bit of space to store the index and a bit of work for the RDBMS to maintain the index. The index is especially prone to consume space because your transaction_id is such a wide string.

You might consider whether transaction_id really needs to be up to 255 characters long, or if you could declare its max length to be something shorter.

Or you could use a prefix index to index only the first n characters:

CREATE INDEX join_id ON UserTrans (transaction_id(16), source(16));

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@Daniel has a good point that you might get the same benefit and save even more space by indexing only one column. Since you’re doing SELECT * you’ve ruled out the benefit of a covering index.

Also if you intend transaction_id to be unique, why not constrain it to be unique?

CREATE UNIQE INDEX uq_transaction_id ON UserTrans (transaction_id(16));