An expression index is a database

Relational database management system

A relational database management system is a database management system that is based on the relational model as introduced by E. F. Codd. Most popular databases currently in use are based on the relational database model....

 index

Index (database)

A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of slower writes and increased storage space...

 that is built on a generic expression, rather than on a list of columns. This allows indexes to be defined for common query conditions that depend on data in a table, but are not actually stored in that table.

A common use for an expression index is to support case-insensitive searching or constraints. For example, if a web site wants to make user names case-insensitive, but still preserve the case as originally entered by the user, a unique index can be created on the upper- or lower-case representation of the user name:

CREATE UNIQUE INDEX site_user__user_name_lower ON site_user( lower( user_name ) );

That will create a unique index on "lower(user_name)". Any queries that search on "lower(user_name)" could then make use of that index:

SELECT user_id FROM site_user WHERE lower(user_name) = lower('Decibel');