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AUTO_INCREMENT Increasing by Multiples¶
Length: 995 words; Published: October 22, 2019; Category: Schema & SQL; Type: Troubleshooting
The numeric value of a column which is a key index and uses the AUTO_INCREMENT attribute, will be increased automatically. Normally, it’s increased by a factor of one. So, for the first row, the key auto_increment column should have a value of 1, the second a value of 2, and the third a value of 3.
However, when using Galera, an auto_increment column will increase by a greater amount. This isn’t a problem, but it’s something you need to understand and it’s something of which you need to be aware.
Scenario
Suppose we’re developing a database for a store which sells many things, including toys. Related to this, we decide to create a simple table called, toys. Our intention is to store a list of toys that we sell, including how many we have in stock and the prices.
Below is a description of this new table:
DESC toys;
+----------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+----------------+
| toy_id | int(11) | NO | PRI | NULL | auto_increment |
| toy_name | char(25) | NO | | | |
| quantity | int(11) | NO | | 0 | |
| price | decimal(6,2) | NO | | 0.00 | |
+----------+--------------+------+-----+---------+----------------+
As you can see, the first column, called toy_id, uses auto_increment. Let’s see what happens when three rows of data are inserted into the table. Keep in mind that this is a new table which has never contained data.
INSERT INTO toys
VALUES(NULL, 'Baseball', 12, 6.15),
(NULL, 'Frisbee', 6, 12.45),
(NULL, 'Slinky', 8, 6.95);
SELECT * FROM toys;
+--------+----------+----------+-------+
| toy_id | toy_name | quantity | price |
+--------+----------+----------+-------+
| 3 | Baseball | 12 | 6.15 |
| 6 | Frisbee | 6 | 12.45 |
| 9 | Slinky | 8 | 6.95 |
+--------+----------+----------+-------+
Look at the values for the toy_id column. Instead of having values of 1, 2, 3, they have values of 3, 6, 9. They’re incrementing by a factor of three. The first time encountering this, it may seem to be a problem, possibly a bug in the software. It’s not. It’s intentional, by design.
Explanation
In a Galera Cluster, all nodes may write data to the tables. Imagine a situation in which all nodes in the cluster simultaneously try to insert rows in the same table at the same time. The result could potentially be duplicate values for any columns which use auto_increment. To avoid such conflicts, Galera increments values for the columns based on the number of nodes in the cluster.
Let’s see how many there in the cluster we’re using. We would enter the following SQL statement to get the number of nodes in the cluster:
SHOW STATUS LIKE 'wsrep_cluster_size';
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| wsrep_cluster_size | 3 |
+--------------------+-------+
There are three nodes. This explains why the toy_id column is incrementing 3 at a time. It doesn’t explain why the first three rows didn’t have values of 1, 4, and 7. The reason for that is each node has a different starting point. The node on which the INSERT statement above was entered, happened to have been the third node to join the cluster. Let’s get a list of variables related to auto_increment. It’ll make more sense.
SHOW VARIABLES LIKE '%auto_increment_%';
+------------------------------+-------+
| Variable_name | Value |
+------------------------------+-------+
| auto_increment_increment | 3 |
| auto_increment_offset | 3 |
| wsrep_auto_increment_control | ON |
+------------------------------+-------+
The auto_increment_increment variable indicates that the node is set to increase any auto_increment field by a factor of 3. The auto_increment_offset variables provides it’s starting point from the value of the last row inserted. Notice that wsrep_auto_increment_control is enabled. This feature is what adjusts these other two variables based on the number of nodes in the cluster. If a node leaves the cluster, it will adjust them accordingly.
If we were to get the AUTO_INCREMENT value on each of the three nodes for the toys table, the value would be different for each. Below shows the results for the initial node in the cluster:
SELECT AUTO_INCREMENT
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_SCHEMA = 'store'
AND TABLE_NAME = 'toys';
+----------------+
| AUTO_INCREMENT |
+----------------+
| 10 |
+----------------+
The value of the second node in the cluster is 11; for the third it’s 12. These values are set as the starting point for each column, the value that will be given to the next row entered from each respective node. With these starting points in mind, let’s look at the auto_increment system variables on the node which started the cluster:
SHOW VARIABLES LIKE '%auto_increment_%';
+------------------------------+-------+
| Variable_name | Value |
+------------------------------+-------+
| auto_increment_increment | 3 |
| auto_increment_offset | 1 |
| wsrep_auto_increment_control | ON |
+------------------------------+-------+
You can see here that the auto_increment_offset on this node differs from the third node in the cluster. On the second node in the cluster, it has a value of 2.
Let’s add three more rows to the table, but this time from the first node:
INSERT INTO toys
VALUES(NULL, 'Ping Pong Paddle', 4, 18.95),
(NULL, 'Gumby & Pokey', 3, 10.25),
(NULL, 'Etch-A-Sketch', 2, 14.25);
SELECT * FROM toys;
+--------+------------------+----------+-------+
| toy_id | toy_name | quantity | price |
+--------+------------------+----------+-------+
| 3 | Baseball | 12 | 6.15 |
| 6 | Frisbee | 6 | 12.45 |
| 9 | Slinky | 8 | 6.95 |
| 10 | Ping Pong Paddle | 4 | 18.95 |
| 13 | Gumby & Pokey | 3 | 10.25 |
| 16 | Etch-A-Sketch | 2 | 14.25 |
+--------+------------------+----------+-------+
Notice that the first new row has a toy_id of 10, which matches the AUTO_INCREMENT value for the table. For the next row, it jumps to 13, and then 16. It doesn’t have the symetry of the results of the first INSERT statement, but it’s logical.
Let’s insert two more rows, but on the second node:
INSERT INTO toys
VALUES(NULL, 'Tonka Dump Truck', 2, 24.95),
(NULL, 'Airport Playset', 1, 18.95);
SELECT * FROM toys LIMIT 6, 2;
+--------+------------------+----------+-------+
| toy_id | toy_name | quantity | price |
+--------+------------------+----------+-------+
| 17 | Tonka Dump Truck | 2 | 24.95 |
| 20 | Airport Playset | 1 | 18.95 |
+--------+------------------+----------+-------+
To save space, we used the LIMIT clause to select the last two rows, the two new rows inserted. As you can see, the second node used the next toy_id in the sequence (i.e., 17) and then just three just in case the other two nodes were inserting rows.
The result of setting the value of auto_increment_increment to the number of nodes, and the auto_increment_offset from 1 to the number of nodes, is that there will be no conflicts between the nodes. Just don’t manually change the value of these two variables or use ALTER TABLE to change the value of AUTO_INCREMENT for any table. That would cause problems.
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