You might think that the content of a normal data file should be unrecognized binary text and we cannot really see it except using special tools. Actually, you can see more than you thought.
In this post, I will show you what we can see in a normal data file without using SQL as the interface language.
Actually, I used a very common text editor vi in Linux to inspect a data file named example01.dbf which contains sample schemas and data.
[oracle@test ~]$ cd $ORACLE_BASE/oradata/COMPDB/
[oracle@test COMPDB]$ vi example01.dbf -c "se nu"
Plain Text in Data Files
I highlighted some clear text that I can recognize on the screenshot. In fact, what we see above from line #10 to #13 is a portion of table HR.LOCATIONS. That is to say, the content of a normal data file can be seen by common means. Anyone has the right to access data files is capable of getting a useful copy of them.
Don't be surprised, it has been a long time to be some degree of plaintext in normal data files. If you concern about it, you should go for Oracle Transparent Data Encryption (TDE).
For your reference, I wrote a tutorial about how to enable Oracle TDE on RAC 19c database, which may offer some help.