After reading through many of the questions here about DB schema migration and versions, I’ve come up with a scheme to safely update DB schema during our update process. The basic idea is that during an update, we export the database to file, drop and re-create all tables, and then re-import everything. Nothing too fancy or risky there.
The problem is that this system is somewhat ‘viral’, meaning that it is only safe to add columns or tables, since removing them would cause problems when re-importing the data. Normally, I would be fine just ignoring these columns, but the problem is that many of the removed items have actually been refactored, and the presence of the old ones in the code fools other programmers into thinking that they can use them.
So, I would like to find a way to be able to mark columns or tables as deprecated. In the ideal case, the deprecated objects would be marked while updating the schema, but then during the next update our backup script would simply not SELECT the objects which have been marked in this way, allowing us to eventually phase out these parts of the schema.
I have found that MySQL (and probably other DB platforms too, but this is the one we are using) supports the COLUMN attribute to both fields and tables. This would be perfect, except that I can’t figure out how to actually use it in a meaningful manner. How would I go about writing an SQL query to get all column names which do not contain a comment matching text containing the word ‘deprecated’? Or am I looking at this problem all wrong, and missing a much better way to do this?
Maybe you should refactor to use views over your tables, where the views never include the deprocated columns.