Support for preprocessor in definitions of external tables using csv clause

Hello

Running 12.2.0.1 on Win64.

I like the csv keyword in creating external tables referring to csv-type data. However; I cannot make it work with the preprocessor keyword.

Where, for instance will I add the preprocessor keyword in a definition like this:

create table Ext_t

(

  A    varchar2 (20 char),

  B    varchar2 (20 char)

)

organization external

  (type Oracle_loader

   default directory Dir_1

   access parameters (

     fields csv missing field values are null

   )

   location ('xxx*.csv'))

  reject limit unlimited;

My preprocessor statement would be like this.

preprocessor dir_prep : 'cmd_prep.cmd'

But where do I put it? Before the fields keyword, I would guess, but the syntax is wrong.