Execution Plan

What is an Execution Plan

Postgres has a great ability to show you how it will actually execute a query under the covers. This is known as an execution plan and which is exposed by explain. Understanding this tells you how you can optimize your database with indexes to improve performance. The hard part for most users is understanding the output of these. While there are many pieces involved in understanding here are a few key things most developers should know.

More About Explain

Every query within Postgres has an execution plan when executed. There are three forms of running explain to expose this to you:

  • The generic form (only shows what is likely to happen)
  • Analyze form (which actually runs the query and outputs what does happen)
  • Verbose form (stay away)

Most commonly, explain is run on SELECT statements. However, you can also use it on:

  • INSERT
  • UPDATE
  • DELETE
  • EXECUTE
  • DECLARE

Using Explain

Given a query:

SELECT last_name FROM employees where salary >= 50000;

We can inspect how Postgres will execute it with:

EXPLAIN SELECT last_name FROM employees where salary >= 50000;
                      QUERY PLAN                          

 Seq Scan on employees  (cost=0.00..16.50 rows=173 width=118)
   Filter: (salary >= 50000)

We can both execute the query and inspect the path/time it took with:

EXPLAIN ANALYZE SELECT last_name FROM employees where salary >= 50000;
                                        QUERY PLAN                                               
--------------------------------------------------------------------------------------------------------
 Seq Scan on employees  (cost=0.00..16.50 rows=173 width=118) (actual time=0.018..0.018 rows=0 loops=1)
   Filter: (salary >= 50000)
 Total runtime: 0.053 ms

Understanding Execution Plans

Perhaps the hardest part of execution plans is actually understanding what they mean. First a brief disclaimer, this is not to be an end all be all reference, but rather a basic starting place to understanding the queries and optimizing your database.

If you run an EXPLAIN ANALYZE on the above table with 2 million rows you might see something like:

image

But lets take a look at what it actually means

image

There's a couple of key items here. Often times you want to look for when a sequential scan is occurring, but more importantly you want to look at what the three items above are. The startup time, the maximum time and finally the number of rows returned. In this case, because we ran EXPLAIN ANALYZE, we have not only the estimated on the left, but the actual on the right as well:

image

In this case we see there's a high time spent and a sequential scan. As a result we may want to try to add an index and examine the results:

CREATE INDEX idx_emps on employees (salary);

With this we've now cut our query time from 295 ms to 1.7 ms:

image