sql_mode must be one of the most elusive issues in setting up a MySQL database. It is also responsible for difficulties in migration, upgrading and securing of databases.
MySQL usually has little issues with migrating database from one machine to another, one version to another, one OS to another, one architecture to another. If we’re not sure, we can always migrate using mysqldump’s logical dump, right?
Not entirely right. The elusive sql_mode (empty by default) may affect out backup+restore, may affect our application behavior, may affect our data integrity.
- Did we remember to set NO_AUTO_CREATE_USER? Oh dear, we have some users without passwords.
- Did we remember to set NO_AUTO_VALUE_ON_ZERO? Oh dear, we’ve dumped our database for backup, restored, but AUTO_INCREMENT values have changed!
- Did we set STRICT_ALL_TABLES? How do we know if the 255 value in our TINYINT really stands for 255 or if it was a truncated 299?
- Do we allow selecting non aggregated columns in GROUP BY? Did we set ONLY_FULL_GROUP_BY? Will our application crash now?
- Our old database has zero for empty date values (columns are NOT NULL). But what settings do we have for NO_ZERO_IN_DATE on our new installation? Will import fail?
- And how did the NULL get in? Was it because we divided by zero, and forgot to set ERROR_FOR_DIVISION_BY_ZERO? How can we tell?
The fact is: two mysql instances, same version, same OS and architecture, with different sql modes – can be incompatible!
As said, the sql_mode is empty by default. This is very non-strict. But more than that: it can be changed even while the database is running; even on a per connection basis.
Setting sql_mode should be one of the first things to do after installation. The usual manuals talk about setting the innodb_buffer_pool_size and the query_cache_size, when sql_mode will dictate the nature of your database and application on an entirely grander scale.
I think it would be best if MySQL chooses a desired set of sql modes (like ‘TRADITIONAL’), then make it the default. I further believe it would be best if MySQL would not allow changes to sql_mode. Not globally and certainly not per session. Choosing the stricter mode is better, I believe: errors such as overflow values should be reported to the application, not just swiped under the carpet.
Backward compatibility? Indeed a problem (inherent to the very existence of sql_mode). Perhaps allow one setting per installation. Perhaps just go for it.
sql_mode was one of the first things tossed in Drizzle. It was a horrible idea to begin with and it just kept making everything worse.
Well I think the SQL_MODE was mainly invented as a BC measure.
And yes being able to set it in the session is problematic. I filed a bug on this long ago. For example its possible to change the setting for invalid dates in a session. Ugh!
Lukas,
Exactly so. I was just sitting down to write about exactly that: the fact that any user can use sql_mode and break data integrity (to be honest, sql_mode is only one of several such vulnerabilities). I’ll post sometime next week.
I was wondering about the origins of sql_mode. Seemed to me more like a bait with which to lure DBAs from other RDBMS. I think this reasoning explains the ORACLE, DB2, POSTGRESQL sql_modes.
Brian, good to know. I have a feeling that in retrospection the MySQL developers would not have invented it themselves.
I guess this is just one of those historic codes you never thought would go that far.
Thanks,
Shlomi
I disagree – being able to override the server’s sql_mode at the session level is a good thing. This allows backward compatibility without holding back other users and schemas to benefit from a more strict mode.
The only bad thing is that setting the sql_mode on the session level is not privileged. It should have been tied to the privilege system so that it would be possible to disallow changing the mode for particular end users.
Hi Roland,
“without holding back other users and schemas to benefit from a more strict mode”
The other users cannot benefit from a strict mode if any user can choose to ignore the strict mode.
If it were only allowed for a user to be more strict than the global sql_mode, it would be fine. I think the global sql_mode should be as strict as possible, so no need for that feature as well.
Apart from some minor modes, such as “||” for string concatenation, I don’t see the benefit of allowing users to change settings. If some 3rd based application requires a less strict mode, because it needs to have 0 in dates, then either I should allow 0 in dates globally or I should not use the application. Having it both ways means trouble for everyone.
As a compromise, we may demand that the 0 in dates is only allowed on a particular schema or set of schemas, in which case we will need a priv_zero_in_date privilege in the mysql.db table.
If I’m not mistaken, the latest connectors (connector/J), for example, do not allow 0 in dates, regardless of sql_mode. I need to check on that, though.
Regards,
Shlomi