Path deduplication in oc_filecache

[taminob]

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Is your feature request related to a problem?
My small/medium Nextcloud instance (~80GB) has a database of 300MB, mainly caused (>60%) by the table oc_filecache.
I think there is potential to reduce the database size which might also improve performance for large instances.

Describe the solution you'd like
While working on #41321, I noticed that oc_filecache contains the full (internal) path for each file and additionally the file name.
A path deduplication in the database could decrease the table size by a lot.

This could be achieved by creating a table oc_directories (or oc_paths) containing directories and mapping them to an id.
This id can then be used in the oc_filecache instead of the raw string and the entire path can be re-created by joining oc_filecache and oc_directories and combining the directory path with the file name.
Then, the full path to the directory will only be in the oc_directories table and a directory with lots and lots of files wouldn't increase the table size by that much.

Describe alternatives you've considered

• One minor fix could be to at least drop the file name column (since the information can easily be retrieved from the path using basename).
• The table already contains the column parent with the file_id of the parent directory. Resolving that recursively until parent = file_id could already replace the path column. However, I'm not sure how that could affect the performance for very deep directory nestings and I feel like the solution mentioned above might be a compromise.

Additional context
I can help implementing this, but would appreciate a few pointers if there is something to consider.

I'm also not sure if 3rd-party apps use the filecache - if so, this change would have to be rolled out in a major release since it could break these apps.