[couchand]

The term "flat file database" conventionally refers to a database that is serialized to a single file, whether it is plain text or a binary. The emphasis here is on the single file aspect: it's really easy to transfer the entire database between machines.

[jmulho]

Another meaning of "flat file database" is non-relational -- the kind of thing you might create if you were unaware of normal form and tried to cram the entire data model into a single table. This single table can reside in a single flat file. The file is flat as in not related to any other files. Here is an example "flat file" data model:

emp_id emp_fname emp_lname emp_phone1 emp_phone1_type emp_phone2 emp_phone1_type dept_id dept_manager dept_manager_phone1 dept_manager_phone1_type

This approach has all kinds of problems (solved 40+ years ago by Codd and others). Examples: how do you add a third phone for an employee. How do you keep the managers phone from getting out of sync across rows.

I took "flat JSON file database" to mean essentially the same thing. Something like this:

emp_id emp_JSON_object

JSON perhaps solves some issues (e.g. adding a third phone for an employee), but still suffers from most of the issues (e.g. keeping the managers phone from getting out of sync across employees). Plus it suffers from new issues, namely the fact that each JSON object could have its own layout (e.g. one of them could have a third phone) so there is a bunch of parsing overhead compared to say knowing that phone number is at a specific offset on every row.

[mappu]

MyISAM serializes each table to a single file on disk (well, three files, one each for the definition, data and indexes).

Would you consider MySQL to be a flat file database?

[couchand]

I wouldn't, but I'm not really an expert here. If every table is three files, that implies a non-trivial database would be dozens of files.

I suppose the other use of "flat file" I've heard is a flat directory with no sub-directories, but I've never heard that applied to a "flat file database".