Ok, maybe I'm missing something here but I'm looking at various PHP hosting options and I see things like "10 MySQL databases", or 25 or even unlimited.
Now I've worked on sites with an Oracle backend that have 10,000+ concurrent users and we've had... one database.
The idea of a database is, of course, that you can store whatever you want in it. So why is it for MySQL that the number matters? Is there some table, row or overall database limit I'm not aware of (entirely possible)? Or is it a question or concurrent connections? Or some other performance issue (eg sharding)? The sharding aspect seems unlikely because even basic hosting options (ie under $5/month) I see with 10 databases.
If someone could clue me in on this one, it'd be great.
It's mostly a marketing tactic, although there are some technical and historical considerations.
First, apologies if this is obvious, but SCHEMAs are to Oracle as DATABASES are to MySQL (in over simplified terms, a logical collections of tables).
The host is saying you can have XX number of configured logical databases on a server. Lots of web applications need a database to run. Modern web applications like Wordpress, Movable Type, Joomla, etc., will let you name your tables with a custom prefix. However, if an application doesn't have this configuration feature that means you need one database per install. Also, in a similar vein, if two applications have the same table name, they can't coexist in a single database. Lots of early web applications started out like this, so early on number of databases was an important feature to consider.
There's also access and security. While MySQL (and other databases) can be configured to give users fine grained access-control down to the table and column level, it's often easier to create one user who has full permission on a logical Database. This is important to people who sell services but pass off the actual hosting of completed sites/applications to the shared web-host.
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