Stored functions can be called from SQL statements in a manner similar to built-in functions like DECODE, NVL, or RTRIM. This is a powerful technique for incorporating business rules into SQL in a simple and elegant way. Unfortunately, there are a number of caveats and restrictions.
The most notable caveat is that stored functions executed from SQL are not guaranteed to follow the read consistency model of the database. Unless the SQL statement and any stored functions in that statement are in the same read-consistent transaction (even if they are read-only), each execution of the stored function will look at a different time-consistent set of data. To avoid this potential problem, you need to ensure read consistency programmatically by issuing the SET TRANSACTION READ ONLY or SET TRANSACTION SERIALIZABLE statement before executing your SQL statement containing the stored function. A COMMIT or ROLLBACK then needs to follow the SQL statement to end this read-consistent transaction.
The syntax for calling a stored function from SQL is the same as referencing it from PL/SQL:
schema_name is optional and refers to the user/owner of the function or package. pkg_name is optional and refers to the package containing the called function. func_name is mandatory and is the function name. db_link is optional and refers to the database link name to the remote database ...