Indexes in PostgreSQL

This works very similarily to how it works in MongoDB so we're going to breeze over this pretty quickly.

Let's take this query:

SELECT comment_id, user_id, time, LEFT(comment, 20) FROM comments WHERE board_id = 39 ORDER BY time DESC LIMIT 40;

This should be a pretty common query if we're making a message board: grab all the comments for a particular board. Let's see what PostgreSQL does under the hood by adding an EXPLAIN in front of it.

EXPLAIN SELECT comment_id, user_id, time, LEFT(comment, 20) FROM comments WHERE board_id = 39 ORDER BY time DESC LIMIT 40;

This part should break your heart: Seq Scan on comments. This means it's looking at every comment in the table to find the answer. This is a place we'd need an index to prevent this. Let's make an index on board_ids to speed this up.

CREATE INDEX ON comments (board_id);
EXPLAIN SELECT comment_id, user_id, time, LEFT(comment, 20) FROM comments WHERE board_id = 39 ORDER BY time DESC LIMIT 40; -- run again

If you're looking the EXPLAIN again, you'll see it does a Bitmap Heap Scan instead of a Seq Scan. Much better!

Let's do one more; all users should have a unique username. Let's ensure that with a unique index.

CREATE UNIQUE INDEX username_idx ON users (username);
INSERT INTO users (username, email, full_name, created_on) VALUES ('aaizikovj', 'lol@example.com', 'Brian Holt', NOW()); -- this will fail

• The username_idx is just a name for the index. You can call it whatever you want.
• Try inserting a duplicate username. The query will fail.
• A pleasant byproduct is that this field is now indexed so you can easily search it.

PostgreSQL has more types of indexes. Feel free to explore them more.