Question

When an oracle explained plan is consider good?
I’m try to refactor a DB Schema, and there are so many query in view and packages that are so slow.

For example, this is one of the most orrible query, and give me this explain plan:

Plan
ALL_ROWSCost: 18,096 Bytes: 17 Cardinality: 1

I don’t ask how to fix a query, just how to consider the explain plan as good. Thanks!!

Answer 1

Before considering the result of an Explain Plan we need to understand following terminologies,
Cardinality– Estimate of the number of rows coming out of each of the operations.
• Access method – The way in which the data is being accessed, via either a table scan or index
access.
• Join method – The method (e.g., hash, sort-merge, etc.) used to join tables with each other.
• Join type – The type of join (e.g., outer, anti, semi, etc.).
• Join order – The order in which the tables are joined to each other.
• Partition pruning – Are only the necessary partitions being accessed to answer the query?
• Parallel Execution – In case of parallel execution, is each operation in the plan being
conducted in parallel? Is the right data redistribution method being used?

By reviewing the four key
elements of: cardinality estimations, access methods, join methods, and join orders; you can determine if the execution plan is the best available plan.
This white paper will help you, http://www.oracle.com/technetwork/database/focus-areas/bi-datawarehousing/twp-explain-the-explain-plan-052011-393674.pdf