Add parent index to oc_shares table

[dtygel]

Steps to reproduce

1. Have more than 100 shared files to circles in a directory A
2. Go to apps/files and go to directory A

Expected behaviour

See the list of files in a reasonable time

Actual behaviour

It takes ages to load the files' list. Looking at long queries in mysql log, it's because the field parent is not indexed.

We've tested locally, and had an increase in performance from 1.8s for the query to 0.4s, a huge improvement by just adding the field parent as a normal index.

We don't know how to add the index to the core tables. If we can get some reference on how to do this (table oc_shares), we can submit a simple Pull Request.

[MorrisJobke]

Could you run occ db:add-missing-indices and check if this already helps with your problem? Otherwise @rullzer can also have a look at this.

[rullzer]

I'll take care of this!