Add a note about correct cfg value server_version for MariaDB

[tristanbes]

Because when using a MariaDB database, you should prefix the server_version with mariadb-otherwise doctrine/dbal is not able to recognize it and use MySQL57Platform which results in:

• possible bugs
• endless schema diff generation

Because this feature/bug is not documented, it is time to do it properly 📦 👍

• Discussion on Bug with server_version for MariaDB >10.2.7 doctrine/dbal#2985 (comment)
• Also, why it should be prefixed when using MariaDB: https://github.com/doctrine/dbal/blob/f76bf5ef631cec551a86c2291fc749534febebf1/lib/Doctrine/DBAL/Driver/AbstractMySQLDriver.php#L135

[javiereguiluz]

Thanks for proposing this fix! I have some questions about this:

• Is this the version number used by MariaDB itself (to differentiate from MySQL) or is this just a Doctrine feature?
• If it's a Doctrine feature, is it a temporary solution until Doctrine 3 is released or is it definitive?

I ask this because, even if I disagree with lots of the decisions made by Doctrine members, I admit that they always try to do things technically correct and this solution feels a bit "hacky". My main concern is that the $version option now behaves differently depending on your database: most of the times is just the "version number" but other times is the "vendor name" + the "version number".

Thanks!

[tristanbes]

The version itself (10.2.12) is the version from MariaDB server, but the prefix (mariadb-) seems to be a Doctrine feature to differentiate from MySQL.

For the rest of your question I guess you'll have to ask to a Doctrine maintainer

[xabbuh]

small nitpicking: maybe use "e.g." instead of "ex:" for consistency with other parts of the docs

[tristanbes]

done.

[javiereguiluz]

@tristanbes thanks for this contribution! I've merged it so people can avoid this issue, but I've also opened an issue in DBAL repo to see if this can be solved differently (doctrine/dbal#3110). Cheers!