DEV Community


How to modify the size of a column in MySql from command line

ama profile image Adrian Matei Originally published at ・3 min read

This blog post presents the steps required to connect to the MySql database from the command line and modify the size of a column in a table. The example is based on the MySql database that is backing Keycloak to run For a more detailed setup you can see the wiki article Keycloak MySQL Setup

This all began with an error thrown by the Chrome extension - Save to - I was trying to select a very long text to add the bookmark's description. The error in the Keycloak logs was the following:

Caused by: com.mysql.jdbc.MysqlDataTruncation: Data truncation: Data too long for column 'DETAILS_JSON' at row 1
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(
    at com.mysql.jdbc.MysqlIO.sendCommand(
    at com.mysql.jdbc.MysqlIO.sqlQueryDirect(
    at com.mysql.jdbc.ConnectionImpl.execSQL(
    at com.mysql.jdbc.PreparedStatement.executeInternal(
    at com.mysql.jdbc.PreparedStatement.executeUpdateInternal(
    at com.mysql.jdbc.PreparedStatement.executeUpdateInternal(
    at com.mysql.jdbc.PreparedStatement.executeLargeUpdate(
    at com.mysql.jdbc.PreparedStatement.executeUpdate(
    at org.jboss.jca.adapters.jdbc.WrappedPreparedStatement.executeUpdate(
    at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(
    ... 91 more

Fortunately in the this case the error in the logs is clear enough - the column's size seemed too short to handle the bigger selection.

So let's dig in to correct the problem.

Connect to the database

First connect to the database

mysql -u keycloak -p keycloak 


  • keycloak in -u keycloak is the username
  • the last keycloak is the name of the database

You will be asked for the user's password (-p).

Find the right table

The column name DETAILS_JSON was mentioned in logs. To find out which table this columns belongs to I listed the
columns of all tables from the keycloak database:

select * from information_schema.columns
where table_schema = 'keycloak'
order by table_name,ordinal_position;

The result was not easy to interpret - I got a pretty long lists of columns, mainly because Keycloak uses lots of tables. So I needed a way
to grep the result. Apparently you can grep in mysql shell by issuing the following command

pager grep DETAILS_JSON;

and then run the above command again:

mysql> select * from information_schema.columns where table_schema = 'keycloak' order by table_name,ordinal_position;
| def           | keycloak     | EVENT_ENTITY                  | DETAILS_JSON                 |                3 | NULL                        | YES         | varchar   |                     5550 |                   5550 |              NULL |          NULL |               NULL | latin1             | latin1_swedish_ci | varchar(5550) |            |       | select,insert,update,references |                |                       |
519 rows in set (0.00 sec)

I could now clearly identify the table as EVENT_ENTITY

Change the column's size

First we need to disable the grep/pager to be able to see anything else which does not contain the DETAILS_JSON text in it.
You can do that by issuing the following command:

mysql> nopager;

Then display the columns of the EVENT_ENTITY table to make sure the searched column is there:

mysql> show columns from EVENT_ENTITY;
| Field        | Type          | Null | Key | Default | Extra |
| ID           | varchar(36)   | NO   | PRI | NULL    |       |
| CLIENT_ID    | varchar(255)  | YES  |     | NULL    |       |
| DETAILS_JSON | varchar(2550) | YES  |     | NULL    |       |
| ERROR        | varchar(255)  | YES  |     | NULL    |       |
| IP_ADDRESS   | varchar(255)  | YES  |     | NULL    |       |
| REALM_ID     | varchar(255)  | YES  |     | NULL    |       |
| SESSION_ID   | varchar(255)  | YES  |     | NULL    |       |
| EVENT_TIME   | bigint(20)    | YES  |     | NULL    |       |
| TYPE         | varchar(255)  | YES  |     | NULL    |       |
| USER_ID      | varchar(255)  | YES  |     | NULL    |       |
10 rows in set (0.00 sec)

We can see now it has a size of 2550 characters. We'll just more than double that by altering the table:

Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

Run the above command again to make sure the column has now the new size.


This post serves me as a reminder on how to connect to a MySql database and modify a column.

Discussion (0)

Editor guide