Question

mysql will throw an error when the inserted data is larger than the destination column. This seems, all in all, to be a positive behavior.

However, I would like to cancel this behavior for the duration of one specific transaction – in this tx, I want truncation to happen, if the data is too large.

Is there a way to disable this behavior, but only for one transaction? Here is the documentation of how to this is at the JDBC level:

Starting with Connector/J 3.1.0, the JDBC driver issues warnings or
throws DataTruncation exceptions as is required by the JDBC
specification unless the connection was configured not to do so by
using the property jdbcCompliantTruncation and setting it to false.

Answer 1

The behavior depends on the MySQL session variable sql_mode. You can change the variable during a connection, and reset it to the original value later. On my system, the default value of sql_mode is:

SELECT @@sql_mode;
-- REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,ONLY_FULL_GROUP_BY,ANSI,NO_AUTO_VALUE_ON_ZERO,STRICT_ALL_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION,PAD_CHAR_TO_FULL_LENGTH

You can parse this result in your program, remove the item STRICT_ALL_TABLES, update the value with SET SESSION sql_mode='...', execute the statements and reset the variable at the end of the transaction.

Edit by @ripper234:

Here is the code I wrote in spirit of this answer:

private static final disableMysqlStrictMode = 
    ((String)Play.configuration.get("db.url")).contains("mysql://");

...

String originalSqlMode = null;
try {
    if (disableMysqlStrictMode) {
        // Strip away mysql's "strict mode" for this transaction - in case one of the columns is truncated, we don't want the entire tx to fail
        // http://stackoverflow.com/a/10606085/11236
        Query query = JPA.em().createNativeQuery("SELECT @@sql_mode;");
        originalSqlMode = (String)query.getSingleResult();
        String newSqlMode = originalSqlMode
            .replace("STRICT_ALL_TABLES", "")
            .replace("STRICT_TRANS_TABLES", "")
            .replace(",,", ",");
        JPA.em().createNativeQuery("SET SESSION sql_mode=?").setParameter(1, newSqlMode).executeUpdate();
    }


    // Save my entity here
    obj.save();


} finally {
    if (originalSqlMode != null) {
        // Restore original sql_mode
        JPA.em().createNativeQuery("SET SESSION sql_mode=?").setParameter(1, originalSqlMode).executeUpdate();
    }
}

It can be wrapped in a prettier form if one is so inclined.