Can this be fixed?

Question

I'm on 9i R2 Patch 7.
We use CBO and the Statistics on all the Tables are current. It happens daily.
The explain plan for the query below uses a HASH JOIN, does not make use of Indexes and takes 30 seconds to complete.
SELECT Column1, Column2
FROM Table X LEFT OUTER JOIN Table Y ON X.Column = Y.Column
The Explain Plan for the query below uses a NESTED LOOP JOIN, makes use of Indexes and completes in 1 second.
SELECT Column1, Column2
FROM Table X INNER JOIN Table Y ON X.Column = Y.Column
So, the ultimate question is how do I get the CBO to use an Index for the LEFT OUTER JOIN? Why is the LEFT OUTER JOIN so expensive.
Is there a way to fix this? The LEFT OUTER JOIN is required.

Answer 1

Hey dude,
It is true that full table scan is sometimes much cheaper than index scan.
Actually when indexes are used, it will try to acumulate as much memory as possible from database buffer cache. In that process index scans can accumulate all the memory available and the whole application will get screwed up.
But full table scan uses only fixed set of blocks in the memory. CBO will take this into consideration to calculate the cost of the query. So, as the number of records (including rowid scans) increases cost of index scan increases as compared to table scan.
This is vast topic and difficult to understand. But it is true that full table scans can be cheaper costwise compared to index scan. The main reason is when performing full table scans only that query will be slow but wrong index scan can screw up the whole application.
Also please note that there are couple of init.ora parameters where you can change the relative cost between full table scan and index scan.