In SAS , Mainly Data Step Works on Program Data Vector ( PDV) .
-- In datastep, this always works using PDV
In SAS , Then how Proc SQL Works ( backend Operations) ?
-- SAS implements a ANSI parser and compiler which you can access through a procedure. It operates like any other ANSI SQL compiler. You can see it processing by adding the options _tree and _method:
http://www2.sas.com/proceedings/sugi30/101-30.pdf
You will also find a lot of information in the docs:
http://support.sas.com/documentation/cdl/en/sqlproc/63043/HTML/default/viewer.htm#titlepage.htm