[Idea] Add unless index_exists? on index creation

[ohaddahan]

I recently used this gem to add concurrently an index on a large and busy table on Heroku.

The operation lost connection midway but the migration finished.

Seems like some parts of Rails book keeping failed and now I get failing new builds since it think the migration need to be done but the index exists.

I think that adding unless index_exists? to the recommendation will be useful for future similar issues others may have.

[ankane]

Hey @ohaddahan, thanks for the suggestion. I've experienced this as well (and sometimes used the same trick), but I don't want to make it the default recommendation. Sometimes, concurrent indexes fail to build and exist but are invalid. In this case, I think it's better to throw an error so developers can manually resolve the issue (most likely, drop the invalid index and run the migration again).