FAQ: Introduction To Indexes - When should I add an Index?

This community-built FAQ covers the “When should I add an Index?” exercise from the lesson “Introduction To Indexes”.

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This exercise can be found in the following Codecademy content:

Design Databases With PostgreSQL

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Hello any fellow Postgrezzers!

CREATE INDEX customers_last_name_email_address_first_name_idx ON
customers(last_name, email_address, first_name);

I don’t quite understand why the task insists that the order of appearance for the customer columns should go: last_name, email_address, first_name?

Why not first_name, last_name, email_address?

Thanks if anybody can help me understand this.

As decribed in the lesson, searches for customers by last_name are running slowly.
because the main search filter is last_name you want that to be the first thing the index is built upon.

I was wondering how in the 2nd exercise, would option 3

CREATE INDEX customers_last_name_email_address_first_name_idx ON
customers(last_name, email_address, first_name);

be the best choice? email_address is likely to have less records than last_name or first_name, so wouldn’t listing email before last_name yield a quicker result, even when searching by last_name only?

When using the query below, it seems to be faster by a small margin with email_address listed 1st followed by last_name

EXPLAIN ANALYZE VERBOSE SELECT *
FROM customers
WHERE last_name = 'Jones';

Can anyone verify my thinking on this? Thanks!