A guide to using indexes in MySQL

In the previous section, we learned about primary keys and secondary keys, as well as B-trees and how they relate to each other. However, we still don't know how to apply this knowledge to optimize our database performance. In this video, we will dive into how to use indexes effectively to improve query performance.

Start with your queries

We've said before that your queries should drive your indexes. What does this mean? It means that you should start by analyzing the access patterns of your application before deciding where to put indexes.

Before adding an index, ask yourself, which queries are you running frequently and which tables are they accessing? By analyzing the access patterns of your queries, you can better determine which indexes will be most useful. Keep in mind that not all queries require indexes, and adding too many indexes can actually harm your database's performance.

When to use indexes

Here are some general rules of thumb to consider when deciding whether or not to use an index:

Do not assume that anything that shows up in the where clause of a query should have an index. Consider all queries being run and their respective access patterns.
Do not create an index on every column. This will slow down inserts by functionally duplicating your table. It also won't help reads as much as you'd hope.
Do consider the entire query when deciding which columns to index. This includes sorting, grouping, and joining.
Do not worry about trying to create the perfect index for every query. It may not always be possible, and sometimes you will have to rework the queries to take advantage of existing indexes.

Adding an index in MySQL

Here's an example of how to add an index to a table in MySQL:

alter table people add index birthday_index (birthday);

This creates an index named birthday_index on the birthday column of the people table.

Using indexes for specific queries

Let's take a closer look at how indexes can optimize specific queries. We will be using the people table we used in the previous section, which includes an id, first_name, last_name, state, email, and birthday column.

Equality

The most basic use of an index is for direct access, where the query searches for a specific value in a particular column. Here's an example:

select * from people where birthday = '1989-02-14';

Here, we're finding all people whose birthday is February 14th, 1989. Since there is an index on the birthday column, MySQL can use this index to find the rows that match the query, resulting in a low execution time.

Ranges

Indexes can also be used for unbounded and bounded ranges, where a query requests all rows that fall within a certain range. In unbounded ranges, there is either no upper or lower limit on the range. Here's an example:

select * from people where birthday >= '2006-01-01';

This query finds all people born on or after January 1st, 2006. Since there is an index on the birthday column, MySQL can use this index to find the rows that match the query.

In bounded ranges, there are upper and lower limits on the range. Here's an example:

select * from people where birthday between '2006-01-01' and '2006-12-31';

This query finds all people born in the year 2006. We're using the between keyword to set the upper and lower bounds on the range. Once again, since there is an index on the birthday column, MySQL can use this index to find the rows that match the query.

Sorting

Indexes can also be used to sort rows instead of having MySQL do a full table scan. Here's an example of a sorted query:

select * from people order by birthday limit 10;

This query will return the first 10 rows of the people table sorted by the birthday column. Since there is an index on the birthday column, MySQL can use this index for sorting instead of doing a full table scan.

Grouping

Indexes can also be used to group rows together for an aggregate function. Here's an example:

select birthday, count(*) from people group by birthday;

This query groups all people by their birthdays and returns a count of all people born on the same day. Once again, since there is an index on the birthday column, MySQL can use this index to group rows together instead of doing a full table scan.

Conclusion

In this article, we've learned how to use indexes effectively to optimize our database queries. We started by discussing how queries should drive indexes, and provided guidelines on when and how to use indexes. We then explored how to add an index in MySQL, and provided examples of how indexes can optimize specific queries involving equality, ranges, sorting, and grouping.

Remember, indexing is an art that requires skill and time to learn. By following best practices and analyzing your queries and access patterns, you can find the right balance of indexes to optimize your database queries without compromising performance.