Today I Learned

hashrocket A Hashrocket project

Using Expressions In Indexes With PostgreSQL

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('');

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.

See More #sql TILs
Looking for help? Hashrocket developers believe that data quality is as important as code quality. We enjoy all the challenges of relational databases, from finding the fastest index, to structuring data to fit the needs of an application. We're eager to share our experiences; check out PG Casts, our series of free PostgreSQL screencasts.