Should I apply string manipulation after or before joining tables in Oracle

Question

I have two tables need to inner join, one table has relatively small number of records compared to the other one. I need to apply some string manipulation to the smaller table, and my question is can I apply the string function after the join, or should I apply them in a sub query and then join the sub select to the bigger table?

An example would be something like this:

Option 1:

SELECT SUBSTR("SMALL_TABLE"."COL_NAME",x,y) "NEW_COL" FROM "BIG_TABLE"
JOIN "SMALL_TABLE" ON ...

Option 2:

SELECT "NEW_COL"
FROM "BIG_TABLE"
JOIN 
(
SELECT SUBSTR("SMALL_TABLE"."COL_NAME",x,y) "NEW_COL" FROM "SMALL_TABLE"
) "T"
ON ...

Which is better for performance option 1 or 2? I am using oracle 11g.

Answer 1

Regardless of how you structure the query, Oracle's optimizer is free to evaluate the function before or after the join. Assuming that the string manipulation is only done as part of the projection step (i.e. it is done only in the SELECT clause and is not used as a predicate in the WHERE clause), I would expect that Oracle would apply the SUBSTR before joining the tables if you used either formulation because it would then have to apply the function to fewer rows (though it can probably treat the SUBSTR as a deterministic call and cache the results if it applies the function after the join).

As with any query optimization question, the first step is always to generate a query plan and see if the different queries actually produce different plans. I would expect the plans to be identical and, thus, the performance to be identical. But there are any number of reasons that one of the two options might produce different plans on your system given your optimizer statistics, initialization parameters, etc.

Answer 2

It is better to apply the operations before doing the join and then joining and querying for the final result. This is called query optimization. By doing so for ur question you will perform lesser operations when "join"ing as u will be eliminating the useless rows beforehand.
Lots of examples here : http://beginner-sql-tutorial.com/sql-query-tuning.htm
and this is the best one I could find : http://www.cse.iitb.ac.in/~sudarsha/db-book/slide-dir/ch14.ppt‎