Ability to drop table columns using dbDelta()

[toszcze]

dbDelta() function doesn't drop columns that don't exist in the creation queries, but exist in the database.

I've attached a patch that adds this feature. Dropping columns is optional and disabled by default (it can be enabled by using the 'dbdelta_drop_columns' filter).

[nacin]

This feels a bit dangerous to include in such a shaky function.

We actually don't drop columns in WordPress unless we absolutely need to. When we've "dropped" columns in the post table over the years, for example, we've just left them on old installs.

[toszcze]

I agree it's dangerous - that's why this feature should be definitely disabled by default. Personally I think that it could be helpful for plugin developers who use their own database tables, but it's only my humble suggestion.

[nacin]

My point was that dbDelta() is extremely finicky. It requires exacting whitespace, among other things, in order to work. When it breaks and doesn't create a new table, or a new index, or a new field, well, that's not the end of the world. If it breaks and suddenly starts dropping columns because it didn't notice them in the CREATE TABLE, that's very, very bad.

As long as dbDelta() is in the shape it's in, I'm not OK with giving it the ability to destroy data in the process.