PostgreSQL BRIN Indexes: Big Data Performance With Minimal Storage
PostgreSQL 9.5 introduced a feature called block range indexes (aka BRIN) that is incredibly helpful in efficiently searching over large time series data and has the benefit of taking up significantly less space on disk than a standard B-tree index. A block range index entry points to a page (the atomic unit of how PostgreSQL stores data) and stores two values: the page's minimum value and the maximum value of the item to be indexed.
In fact, when used appropriately, a BRIN index will not only outperform a B-tree but will also save over 99% of space on disk!
Source: PostgreSQL BRIN Indexes: Big Data Performance With Minimal Storage, an article by Jonathan S. Katz.