Query with Explain Plan

Question

Suppose, i am using two queries [ which are almost equal] but using separate table name aliases. Will this result is separate Explain Plans?
i.e.
Explain Plan for : Select * from emp a;
and Explain for : Select * from emp b;
Will remain same always, or it will differ?
Also, i was looking for low level conversion of Parsed SQL statements to Machine Level Language. I mean how this step is performed. Where does the compiler/interpreter resides and how it works.
Thanks in Advance.
Regards,
Sudipta.

Answer 1

user13513014 wrote:
Suppose, i am using two queries [ which are almost equal] but using separate table name aliases. Will this result is separate Explain Plans?
i.e.
Explain Plan for : Select * from emp a;
and Explain for : Select * from emp b;
Will remain same always, or it will differ? By simply switching alias, the optimizer is unlikely to develop a new plan.
Also, i was looking for low level conversion of Parsed SQL statements to Machine Level Language. I mean how this step is performed. Where does the compiler/interpreter resides and how it works.
Thanks in Advance.
Regards,
Sudipta.There is no way you can actally see the parsing of SQL. It is done internally by oracle in a process caled Hard Parse or Soft Parse. A Hard Parse is where the optimizer must go through all the steps of parsing from the beggining, where as a soft parse, oracle uses the cached results from a hard parsed query that is in the shared pool.
How the optimizer works is a subject of ample documentation and several books. As a general over view, here are the general steps at parsing an SQL:
1. Optimizer evaluates your statement, checks it for grammar errors, grants to objects, etc.
2. Statement Transformation: Optimizer evaluates your statement for possible optimization regarding rewrite, in cases you have view or correlated subqueries.
3. Choice of optimizer goals.
4. Choice of acccess paths.
5. Choice of Join Orders.
Suggest you try a google search, it will give you a lot of material, or check the Oracle Database Performance Tuning Guide 11.2, which is free by oracle.