til/postgres/using-expressions-in-indexes.md

Using Expressions In Indexes

Though we usually see column names by themselves when defining an index, it is also possible to create an index with an expression.

Let's say I have a users table with an email column. Then I may end up creating an index like this

create index email_idx on users (email);

If I always perform queries on the email column with the lower() function, like this

select * from users where lower(email) = lower('some@email.com');

then I will want to also create an index with that full expression -- lower(email)

I can do this with a statement like the following

create index lower_email_idx on users (lower(email));

Without an index that uses the full lower(email) expression, select statements like the one above will be forced to do full sequential scans instead of indexed scans.