Is a database table required for temporary interfaces with flat file data set source ?

Question

Folks,  this is the situation I have in ODI 11.1.1.7
I have a temporary interface (yellow), called MJ_TEMP_INT,  that pulls data from TWO data sets in the source into a temporary target (TEMP_TARG). The catch is one data set pulls from a from a table whereas the other data set pulls from a flat file.  A union is done on the data sets.
I then create another interface, called MJ_INT, that uses the MJ_TEMP_INT as a source and the target is a real db. table called "REAL_TARGET"
Two questions:
When I execute my second interface  (MJ_INT), I get a message "ORA-00942: table or view does not exist" because it is looking for a real db table TEMP_TARG. Why must I have one ? because I am pulling from a flat file ?
On my second interface (MJ_INT) when I look at the property sheet of my source interface MJ_TEMP_INT (yellow), the checkbox next to "Use temporary interface as Derived table" is DISABLED.  Why ? Is is also because my temporary interface is pulling from a flat file ?
I have attached a file that shows a screen shot of my ODI studio.
By the way,  IF my temporary interface source has only one data set pulling from a db. table into a temporary target table, say called MJ_TEMP2_TARG,  and then when I use this temporary interface as a source to another other real db. target table (REAL2_TARGET),  THEN, every thing works.  ODI does not require me to have a real db. table MJ_TEMP2_TARG and the checkbox for "Use temporary interface as Derived table" is NOT DISABLED and my REAL2_TARGET table gets populated.
Thank you in advance.
M. Jamal.

Answer 1

Thanks SH. I thought so. 
Though I understand the reason to materialize the file in a staging area, but that almost defeats the purpose of having a temporary interface in this case if we have to save the data in a permanent db. table first.  I assume the db. table sticks around and is not automatically dropped once the interface executing ends.  If the db. table sticks around then I also must truncate it first before executing the temporary interface each time. Right ?