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

Looking for things like sequential scans can be somewhat useful, but the reality is in the numbers... except when the numbers are just estimates! What is usually far more useful than looking at a query plan is looking at the actual execution. In Postgres, this is the difference between EXPLAIN and EXPLAIN ANALYZE. EXPLAIN ANALYZE actually executes the query, and gets real timing information for every node. That lets you see what's actually happening, instead of what the planner thinks will happen. Many times you'll find that a sequential scan isn't an issue at all, instead it's something else in the query.

The other key is identifying what the actual expensive step is. Many graphical tools will use different sized arrows to indicate how much different parts of the plan cost. In that case, just look for steps that have thin arrows coming in and a thick arrow leaving. If you're not using a GUI you'll need to eyeball the numbers and look for where they suddenly get much larger. With a little practice it becomes fairly easy to pick out the problem areas.