Could you describe the different modules of DBMS's and their function
(parser, optimizer, executor, buffer manager, storage manager, access
methods etc.)?

Hello Cool_007uk,

To answer your question, I will provide a few good references that
include an overall description of a DBMS as well as top level
explanations of each of these functions:

http://www.swen.uwaterloo.ca/~rekram/files/cs798_assignment_1.htm
http://www.math.uwaterloo.ca/~f2yuan/courses/cs798/ass1/

The first is an architectural description of the MySQL database
system, the second looks at PostgreSQL. Both appear to be written up
in response to a homework assignment at the Univeristy of Waterloo in
Canada. Both include several good illustrations showing the overall
structure as well as the functions involved in common operations.

http://www-users.cs.umn.edu/~shekhar/8701/notes/ch3a.html

A set of briefing or lecture notes for a class at the University of
Minnesota. Text only but covers a few additional functions not
described above.

There are a number of additional references available by searching
with phrases such as
  DBMS parser optimizer executor buffer storage manager
or add the name of a relational database system (e.g., Ingres, Oracle)
to get additional references for a specific DBMS.

If you need additional references or if the response is unsuitable in
any way, please use a clarification request so I can provide the
additional information.
  --Maniac