I'm starting to work on a tar FDW as a proxy for a much more specific FDW. (It's the 'faster to build two and toss the first away' approach - tar lets me get the FDW stuff nailed down before attacking the more complex container.) It could also be useful in its own right, or as the basis for a zip file FDW.
I have figured out that in one mode the FDW mapping that would take the name of the tarball as an option and produce a relation that has all of the metadata for the contained files - filename, size, owner, timestamp, etc. I can use the same approach I used for the /etc/passwd FDW for that.
The problem is the second mode where I pull a single file out of the FDW. I've identified three approachs so far:
1. A FDW mapping specific to each file. It would take the name of the tarfile and the embedded file. Cleanest in some ways but it would be a real pain if you're reading a tarball dynamically.
2. A user-defined function that takes the name of the tarball and file and returns a blob. This is the traditional approach but why bother with a FDW then? It also brings up access control issues since it requires disclosure of the tarball name to the user. A FDW could hide that.
3. A user-defined function that takes a tar FDW and the name of a file and returns a blob. I think this is the best approach but I don't know if I can specify a FDW as a parameter or how to access it.
I've skimmed the existing list of FDW but didn't find anything that can serve as a model. The foreign DB are closest but, again, they aren't designed for dynamic use where you want to do something with every file in an archive / table in a foreign DB.
Is there an obvious approach? Or is it simply a bad match for FDW and should be two standard UDF? (One returns the metadata, the second returns the specific file.)
Thanks,
Bear
