Question

I have an InooDB table:

• id – primary key, auto increment
• short_code – varchar(6), unique
• some other columns

The short_code is always generated in php based on id.
The problem is that I can’t generate the short_code before inserting a record, because I don’t know the id.

I tried the following:

1. insert a record with empty short_code
2. get the id of the inserted row with mysqli_insert_id, or pdo, whatever
3. generate the short code based on the id, and update the record with the new short_code

This works, but this is really slow. I have to do an insert and also an update, which is time consuming.

A better idea is to “guess” the id the record I want to insert will have, with
select max(id)+1 from mytable; (or something similar), generate the short_code, and insert the record. Here I have a SELECT and one INSERT which is actually much better.

My question is:

1. Is there a smarter solution for this problem?
2. If no, and I go with solution 2 (guessing the next id), how can I make sure that in the time I do the select max(id)+1 from mytable;, generate short code, and insert a record, nobody will insert another record, so max(id)=1 doesn’t change in meantime?

NOTE!

• I can not generate the short code in a mysql procedure.
• Short code must be generated based on the id.

Answer 1

Auto-inc value is generated when insert occurs, with a locking technique (to avoid generation on two same values) that depends on configuration and you cannot guess it. In a transactional environment many transaction can require a new id, but they may or may not commit, so at the time of the query, the value is reserved, but still not used.

If you want to “guess” the correct value, you have to lock the entire table for write, to avoid another thread to insert a record and to break your guess. This will work if you don’t have a lot of writes on the table, e.g. for few dozen inserts per second.

If there are a lot of updates on this table and write lock is not an option, you can use another table to generate the values (it will have only the id field), and instead of insert/update you will have insert/insert, assuming that the 2 operations will be as fast as they can.