How do you interpret a query's explain plan?

Question

When attempting to understand how a SQL statement is executing, it is sometimes recommended to look at the explain plan. What is the process one should go through in interpr

Answer 1

Basically, you take a look at each operation and see if the operations "make sense" given your knowledge of how it should be able to work.

For example, if you're joining two tables, A and B on their respective columns C and D (A.C=B.D), and your plan shows a clustered index scan (SQL Server term -- not sure of the oracle term) on table A, then a nested loop join to a series of clustered index seeks on table B, you might think there was a problem. In that scenario, you might expect the engine to do a pair of index scans (over the indexes on the joined columns) followed by a merge join. Further investigation might reveal bad statistics making the optimizer choose that join pattern, or an index that doesn't actually exist.