Updated October 13, 2019 to mention that the phpbb_migrations table is always changed with an update.
When updating phpBB, you should backup your database. That’s the guidance given by the phpBB Group. Updates occur when you are moving from one micro version of phpBB to another, say 3.2.6 to 3.2.7. When moving from minor versions, like 3.1 to 3.2, you are doing an upgrade and different procedures apply.
It’s never a bad idea to backup your database. It can be done manually in phpBB: ACP > Maintenance > Backup. This method though is not foolproof. Particularly on larger forums or on shared hosting, backups can time out. Sometimes you think you have a complete backup and you don’t. The only way to know for sure is to download it, and examine it in an editor. If the bottom of the file shows the phpbb_zebra table is being populated and ends with a semicolon, it should be complete. Web hosts often offer features that make automatic database and file backups for you.
But it turns out that when making updates, generally not much of your forum’s database actually changes. Only two tables are guaranteed to change: phpbb_config and phpbb_migrations. (I use phpbb_ as your table prefix. Yours may be different. You can see your actual table prefix in your forum’s config.php file.) This is because phpBB always tracks the current version of phpBB installed, and it is stored as a row in this table. And with each update, one or more rows are added to the phpbb_migrations table to indicate what migrations were made.
Most updates don’t affect many tables. In fact, it’s typical that only the phpbb_config and phpbb_migration tables are affected.
So there is no point in making a full backup of your database prior to updating if you don’t need to. It’s a real hassle on larger forums.
It turns out that if you look through phpBB’s code you can figure out which tables are updated. The /phpbb/db/migration folder contains a number of subfolders. The programs in these subfolders do the work of changing your database during updates.
For example, there is a v320 folder. This contains the migration programs used to bring phpBB up to version 3.2.0.
There is also a v32x folder. This contains the migration programs used for each version update of phpBB 3.2. You can infer from the file names which these are. For example, v321.php and v321rc1.php are used to update from phpBB 3.2.0 to 3.2.1. “rc” in the file name means “release candidate”. There is typically at least one release candidate before an official release. So it’s a matter of reading these programs to see what tables are actually affected.
That’s what I did, and you can see the results on my Do I need to update page. Or you can examine them yourself. I’ll keep the update page updated as new micro releases occur. You might want to bookmark it and refer to it before doing your own update.
If you do it yourself, follow the official procedures on phpBB’s update page. But you may decide to backup only the tables that are actually changed to save yourself extra hassle. If an error occurs, restore these tables and recover your files.