I have a table with 10 columns and I wish to select column 1 and column 9 from the table. In RDBMS how many columns shall be selected internally?
One of the fundamental ideas behind the Relational Model is that RDBMS users characterize their problems in terms of tables & queries representing abstract application relationships and describing abstract application states while the RDBMS interface hides (as much as possible) table & query implementation by another such "logical" RDBMS layer or by other, "physical", paradigm layers. (Hence, logical and physical data independence.)
Your question can only be answered about a particular implementation of a particular version of a particular DBMS. You can find DBMS implementation discussed in textbooks & slides, of which there are numerous sites & .pdfs online.
If this is a performance concern: It just isn't the sort of thing you should worry about until you are familiar with schema design and querying as well as basic performance issues like indexing. Googling: Oracle's .pdf Oracle Database SQL Tuning Guide. Bookboon's free online downloadable ebook Database Design and Implementation: A practical introduction using Oracle SQL.