MySQL get next unique value without auto increment I have an MySQL database (InnoDB) that I'm developing for tracking Work Orders at my organization.  There are multiple 'sites', and each site has work order numbers starting at 100.  

WorkorderID    SiteID    SiteWorkorderNum
     1           1            100
     2           1            101
     3           2            100


WorkorderID is the auto increment field.
SiteID and SiteWorkorderNum are both set as a unique constraint.

Whenever a new work order is about to be inserted for a specific site, the application checks for the max(SiteWorkorderNum) for the siteid of whoever is logged in and returns that value to the insert statement.  The problem I want to avoid is if two users of the same site both enter a new work order at the exact same time.  

I have a couple of possible solutions but I want to see if there is a better way as each has it's drawbacks, but one I'm sure will be better than the other, and of course I'm open to new ideas.

1) Lock the table to guarantee no other users retrieve a SiteWorkorderNum value until after we have retrieved and inserted our values (however let's pretend we have thousands of users trying to enter work orders every minute, wouldn't the table locks slow things down?)

2) Don't lock the table, and retrieve the next available SiteWorkorderNum and try to insert into the database, if I receive a unique constraint error, catch the error and try again until I'm successful.  (this might keep the table freed up, but again pretending we have thousands of users at the same site, would the multiple database calls be a little inefficient?)

Am I being too paranoid about how either of these solutions 'could' effect performance?  Of course I don't have thousands of users, but I'm wanting to apply best practice in this design in case I ever work on a project which does have high traffic.
What is interesting about this situation can be solved using the MyISAM storage.

I answedred a question like this back in April 2012 : How can you have two auto-incremental columns in one table? You need to create one table whose sole purpose is the create sequences of work order for each site

CREATE TABLE site_workorder_seq
(
    SiteID int not null,
    SiteWorkorderNum int not null auto_increment,
    PRIMARY KEY (SiteID,SiteWorkorderNum)
) ENGINE=MyISAM;


Here is a sample loading into this table:

mysql&gt;     DROP DATABASE david;
Query OK, 1 row affected (0.01 sec)

mysql&gt;     CREATE DATABASE david;
Query OK, 1 row affected (0.00 sec)

mysql&gt;     USE david
Database changed
mysql&gt;     CREATE TABLE site_workorder_seq
    -&gt;     (
    -&gt;         SiteID int not null,
    -&gt;         SiteWorkorderNum int not null auto_increment,
    -&gt;         PRIMARY KEY (SiteID,SiteWorkorderNum)
    -&gt;     ) ENGINE=MyISAM;
Query OK, 0 rows affected (0.01 sec)

mysql&gt;     INSERT INTO site_workorder_seq (SiteID) VALUES
    -&gt;     (1),(1),(2),(3),(3),(3),(3),(4),(4),(4),
    -&gt;     (5),(5),(4),(2),(2),(2);
Query OK, 16 rows affected (0.00 sec)
Records: 16  Duplicates: 0  Warnings: 0

mysql&gt;     SELECT * FROM site_workorder_seq;
+--------+------------------+
| SiteID | SiteWorkorderNum |
+--------+------------------+
|      1 |                1 |
|      1 |                2 |
|      2 |                1 |
|      2 |                2 |
|      2 |                3 |
|      2 |                4 |
|      3 |                1 |
|      3 |                2 |
|      3 |                3 |
|      3 |                4 |
|      4 |                1 |
|      4 |                2 |
|      4 |                3 |
|      4 |                4 |
|      5 |                1 |
|      5 |                2 |
+--------+------------------+
16 rows in set (0.00 sec)

mysql&gt;


Let's look at the the last WorkorderNum from each site

mysql&gt; SELECT SiteID,MAX(SiteWorkorderNum) SiteWorkorderNum
    -&gt; FROM site_workorder_seq GROUP BY SiteID;
+--------+------------------+
| SiteID | SiteWorkorderNum |
+--------+------------------+
|      1 |                2 |
|      2 |                4 |
|      3 |                4 |
|      4 |                4 |
|      5 |                2 |
+--------+------------------+
5 rows in set (0.05 sec)

mysql&gt;


Now, suppose you want to get the next SiteWorkorderNum for SiteID 3. You could do this:

INSERT INTO site_workorder_seq (SiteID) VALUES (3);
SELECT MAX(SiteWorkorderNum) INTO @nextworkordernum
FROM site_workorder_seq WHERE SiteID=3;
SELECT @nextworkordernum;


Let's run this and see what happens

mysql&gt; INSERT INTO site_workorder_seq (SiteID) VALUES (3);
Query OK, 1 row affected (0.00 sec)

mysql&gt; SELECT MAX(SiteWorkorderNum) INTO @nextworkordernum
    -&gt; FROM site_workorder_seq WHERE SiteID=3;
Query OK, 1 row affected (0.00 sec)

mysql&gt; SELECT @nextworkordernum;
+-------------------+
| @nextworkordernum |
+-------------------+
|                 5 |
+-------------------+
1 row in set (0.03 sec)

mysql&gt; SELECT * FROM site_workorder_seq;
+--------+------------------+
| SiteID | SiteWorkorderNum |
+--------+------------------+
|      1 |                1 |
|      1 |                2 |
|      2 |                1 |
|      2 |                2 |
|      2 |                3 |
|      2 |                4 |
|      3 |                1 |
|      3 |                2 |
|      3 |                3 |
|      3 |                4 |
|      3 |                5 |
|      4 |                1 |
|      4 |                2 |
|      4 |                3 |
|      4 |                4 |
|      5 |                1 |
|      5 |                2 |
+--------+------------------+
17 rows in set (0.00 sec)

mysql&gt; SELECT SiteID,MAX(SiteWorkorderNum) SiteWorkorderNum
    -&gt; FROM site_workorder_seq GROUP BY SiteID;
+--------+------------------+
| SiteID | SiteWorkorderNum |
+--------+------------------+
|      1 |                2 |
|      2 |                4 |
|      3 |                5 |
|      4 |                4 |
|      5 |                2 |
+--------+------------------+
5 rows in set (0.00 sec)

mysql&gt;


As long as you use this one MyISAM apart from all InnoDB tables, you can generate workordernums per site to your hearts content.