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

Rules of Thumb

(you probably want to read up on the details too:

• Oracle Docs
• ASKTOM
• SQL Server Docs
  )

Bad

Table Scans of Several Large Tables

Good

Using a unique index
Index includes all required fields

Most Common Win

In about 90% of performance problems I have seen, the easiest win is to break up a query with lots (4 or more) of tables into 2 smaller queries and a temporary table.