Elizabeth Christensen adds an index:
Materialized views are widely used in Postgres today. Many of us are working with using connected systems through foreign data wrappers, separate analytics systems like data warehouses, and merging data from different locations with Postgres queries. Materialized views let you precompile a query or partial table, for both local and remote data. Materialized views are static and have to be refreshed.
One of the things that can be really important for using materialized views efficiently is indexing.
Adding indexes to Postgres in general is critical for operation and query performance. Adding indexes for materialized views is also generally recommended for a few different reasons.
Click through for those reasons. This article illustrates some of the differences between indexed views in SQL Server and materialized views in PostgreSQL.