As I have searched over internet, I found many links which explained what are the differences of relational databases and (flat) file system and look at this topic from different aspects and points of view, like the following resources:
File System vs Databases: it describes the missing relational model features in file system but it doesn't mention the vice-versa features.
As I mentioned above, my question is what are the missing file system features in relational model? in other words, what concepts of file systems like pointers (which is discussed about it here) are ignored to implement in relational model?