How SQL Works

When you fire the SQL Query, Basically there are three steps involved in it.
1) Parse
2) Execution
3) Fetch
In Step(1) : During the parse phase, the SQL statement is passed from user process to Oracle and a parsed representation of the SQL statement is loaded into a shared SQL area. Parsing is the process of
1) Translating SQL statement, verifying it to be a valid statement
2) Performing data distionary lookups to check table and column defination
3) Acquiring parse locks on the required objects so that their defination do not change during the statement's parsing.
4) Checking previleges to access referenced schema objects.
5) Determining the optimal execution plan for the statement.
6) Loading it into a shared SQL area.A SQL statement is parsed only if a shared SQL area for an identical SQL statement does not exist in the shared pool. In this case a new shared SQL area is allocated and the statement is parsed.
Step(2) ...During execute phase, at this point Oracle has all necessary information and resources, so the statement is executed.
Step(3) .. In the fetch phase, rows are selected and ordered(if requested by the query), and each successive fetch retrieves another row of the result, until the last row has been fetched.