PostgreSQL Indexes: First Principles

Posted by Ilija Eftimov on January 30, 2016

We have all heard about indexes. Yeah, that thing that it’s automatically added to the Primary Key column that enables fast data retrieval and stuff. Sure, but have you ever asked yourself if there are multiple types or implementations of indexes? Or maybe, what type of indexes your favourite RDBMS implements? In this blog post, we will take a step back to the beginning, exploring what indexes are, what is their role, types of indexes, metrics and so on. And all of this in PostgreSQL.

What’s the Problem?

When explaining indexes, everyone uses the phonebook analogy. The problem is that the phonebook analogy is so good, thinking of any other one is a waste of time. But, before we jump into indexes, we need to understand the problem that they solve.

So, think of a phonebook. The phonebook is a book with all of the names and phone numbers of the people in a city or a country. Now, let’s say we want to find John Doe’s phone number. Knowing that the phone book is alphabetically ordered we look for the page where the surname Doe is. There, we look for John and his phone number. Good, that is efficient enough for a phonebook.

Now, imagine if our phonebook was not alphabetically sorted? Hell, right? We would need to go through all of the pages, reading every name in the phonebook, until we find our John Doe. This is called sequential searching. You go over all of the records, until you find the person whose phone number you are looking for.

One does not need to be a genius to understand that this is super inefficient. The problem with database tables is that the data in them is unordered. If we had a people table, containing all information about people, to lookup the person with the full name John Doe, we would need to execute:

SELECT * FROM people WHERE first_name = "John" and last_name = "Doe";

Easy enough. But although this query is fast, under the hood, the database will hit every row of the table, until it finds the record it is looking for. I am sure your side-project won’t have this problem soon, but imagine a table with millions of records, without indexes. Data retrieval would take seconds, sometimes maybe minutes. Imagine waiting 30 seconds for a list of videos on YouTube?

Introducing Indexes

I guess the phonebook example painted the picture well for you. Now, what’s interesting is that the phonebooks, actually have indexes. But, they are different. They are not like a book index, which shows what chapter starts on which page. Or maybe they do, I haven’t seen a phonebook in a while.

Clustered Indexes

Nevertheless, indexes have different architectures/indexing methods. And the phonebook has a clustered index. Clustering means that the data is in a distinct order, resulting the row data to be stored in order. If this confuses you, think again about the phonebook - the records in the book are ordered alphabetically. Regardless of how simple this might seem to you, it’s a clustered index. The way the data is ordered (clusters) makes it really easy to search and find the needed phone number. So, a clustered index is an index which physically orders the data (the actual bits on disk) in a certain way, and when new data arrives it is saved in the same order.

A caveat with the clustered indexes is that only one can be created on a given database table. That occurs due to their nature - they enforce a data order. Also, clustered indexes increase the write time, because when new data arrives, all of the data has to be rearranged. But on the bright side - they can greatly increase the reading speed of the table.

To summarize - clustered indexes order the data physically (on disk) in clusters.

Non-clustered Indexes

I know you guessed it - if there are clustered indexes, the opposite type has to exist. And you are right, non-clustered indexes are a thing. They are the type that we know to use most, but I guess not everyone knows how their implementation works.

Non-clustered indexes are indexes that keep a separate ordering list that has pointers to the physical rows. It’s basically like a book index, it knows on what page a certain chapter starts and ends. Now, unlike the clustered index, a table can have many non-clustered indexes. But, the caveat with these indexes is that each new index will increase the time it takes to write new records.

So, to summarize - non-clustered indexes do not order the data physically, they just keep a list of the data order.

Indexing effects

So, after we covered the index architecture, we can explore how indexes work in PostgreSQL. But first things first - let’s play with some indexes.

Indexes in PostgreSQL are manipulated via a set of commands. For this example, I will use a table with 1000 records, representing 1000 users. The table schema will look like this:

|             Table "public.users"                |
|  Column    |         Type           | Modifiers |
| id         | integer                | not null  |
| first_name | character varying(60)  |           |
| last_name  | character varying(60)  |           |
| email      | character varying(120) |           |
| gender     | character varying(6)   |           |
| created_at | date                   |           |
|Indexes:                                         |
|   "users_pkey" PRIMARY KEY, btree (id)          |

If you want to follow along with this tutorial, you can easily create this table with the following query:

  first_name VARCHAR(60),
  last_name VARCHAR(60),
  email VARCHAR(120),
  gender VARCHAR(6),
  created_at DATE

Having the table now, we can dump any amount of data we want. I will use the following CSV file, which contains 1000 user records. Dumping the data in the table is easy using PostgreSQL’s COPY command. If you are unfamiliar with the command, check out my post which explains it in depth.

In our case, the actual command is:

COPY users (
FROM '/some/path/here/to/dummy-users.csv'

/* Output: COPY 1000 */

Having the data in the database now, let’s see the real impact of the indexes.

Analyzing Queries

PostgreSQL creates a query plan for each query it receives. When writing your queries, it is very important to choose the best query plan, which adheres to the query structure and the properties of the data. The query plans are really easy to produce in PostgreSQL. It comes with a really neat command called EXPLAIN, which shows the query plan, which contains all of the relevant info.

For example, let’s see the query plan for the query that returns all of the users:


This will return the following output:

                          QUERY PLAN
 Seq Scan on users  (cost=0.00..20.00 rows=1000 width=48)
(1 row)

As you can notice, this simple query requires a Sequential Scan, because there is no WHERE clause in it. This means that it will have to scan each row of the table and return it.

In the parentheses we notice couple of values. The cost is a range of arbitrary units (that closely resemble disk page fetches) - starting from the expected before the output phase can begin, to the estimated total cost of this query. Then, the numbers of rows output that were estimated by the query planner. Last, the width represents the average width of the rows, in bytes.

Let’s add a WHERE clause to the query:

SELECT * FROM users WHERE email = [email protected]';

The query will return the user whose email is [email protected]. Perfect. Let’s EXPLAIN the query, and see its query plan:

EXPLAIN SELECT * FROM users WHERE email = [email protected]';
                          QUERY PLAN
 Seq Scan on users  (cost=0.00..22.50 rows=1 width=48)
   Filter: ((email)::text = [email protected]'::text)

As you can see, although the WHERE clause was added, and the result set was reduced to only one row, the time didn’t go down. Actually, it increased. This happens because the query will issue a Sequential Scan on the table. Let’s see if adding an index to the email column will decrease the cost of the query.

Adding an Index

Adding an index in PostgreSQL is done via the CREATE INDEX command:

CREATE INDEX <index name> ON <table name> USING <method>(<column name>);

Or, in our specific case:

CREATE INDEX email_idx ON users USING btree(email);

After we run this command, let’s see a description of the table:

              Table "public.users"
   Column   |          Type          | Modifiers
 id         | integer                | not null
 first_name | character varying(60)  |
 last_name  | character varying(60)  |
 email      | character varying(120) |
 gender     | character varying(6)   |
 created_at | date                   |
    "users_pkey" PRIMARY KEY, btree (id)
    "email_idx" btree (email)

You can see in the “Indexes” section, the new index appears. After we added the index, let’s see the query plan for the SELECT query:

EXPLAIN SELECT * FROM users WHERE email = [email protected]';
                              QUERY PLAN
Index Scan using email_idx on users  (cost=0.28..8.29 rows=1 width=48)
  Index Cond: ((email)::text = [email protected]'::text)

Whoa! The projected query cost dropped from the original 22.50 to 8.29. So, what changed? As you can notice, instead of a Sequential Scan, now PostgreSQL will issue a Index Scan. We won’t go into details here, because types and approaches of scanning is topic for another blogpost. Simply said, Postgres will issue a scan based on the indexes, therefore finding the exact location of the user that we are looking for and returning it. This is a much cheaper operation then using a sequential scan.

Sequential isn’t Always the Worst

Let’s see a different example. Let’s say, we want to return all of the users that have the id between 100 and 200:

EXPLAIN SELECT * FROM users WHERE id > 100 AND id < 200;
                          QUERY PLAN
Index Scan using users_pkey on users (cost=0.28..11.28 rows=100 width=48)
  Index Cond: ((id > 100) AND (id < 200))
(2 rows)

Looks about right, right? It uses a Index Scan, with an Index Cond(itional) searching for all ids between 100 and 200. Good enough.

How about we try to return all users with ids between 200 and 800?

EXPLAIN SELECT * FROM users WHERE id > 200 AND id < 800;
                          QUERY PLAN
Seq Scan on users  (cost=0.00..25.00 rows=600 width=48)
  Filter: ((id > 200) AND (id < 800))
(2 rows)

Hold on, what happened here? Although there is an index on the primary key column, PostgreSQL decides that doing a sequential scan on the user table is more performant than an index scan. An index scan would require finding 600 indexes and returning each and every one of those records whose index were found. On the other hand, a sequential scan would just go over each of the records and filter out the unwanted rows.

So, although this is a contrived example, you can see in some situations an index scan will not be as performant as a sequential scan.

Careful Usage

Indexes are a powerful way to improve the performance of your tables, but they have to be used carefully. Often, indexes can actually stand in the way of your queries, if not used properly.

Next time we will see what are the index types in PostgreSQL and how we can leverage them.

comments powered by Disqus