BRIN indexes

Block range indexes (BRIN) are of great practical use. All indexes discussed until now need quite a lot of disk space. Although a lot of work has gone into shrinking GIN indexes and the like, they still need quite a lot because an index pointer is needed for each entry. So if there are 10 million entries, there will be 10 million index pointers. Space is the main concern addressed by BRIN indexes. A BRIN index does not keep an index entry for each tuple but will store the minimum and the maximum value of 128 (default) blocks of data (1 MB). The index is therefore very small but lossy. Scanning the index will return more data than we asked for. PostgreSQL has to filter out those additional rows in a later step.

The following example demonstrates how small a BRIN index really is:

test=# CREATE INDEX idx_brin ON t_test USING brin(id); 
CREATE INDEX 
test=# \di+ idx_brin 
                        List of relations 
 Schema |   Name   | Type  | Owner | Table  | Size  
--------+----------+-------+-------+--------+-------+------------- 
 public | idx_brin | index | hs    | t_test | 48 KB
(1 row)

In my example, the BRIN index is 2,000 times smaller than a standard B-tree. The question naturally arising now is: why don't we always use BRIN indexes? To answer this kind of question, it is important to reflect on the layout of BRIN; the minimum and maximum value for 1 MB are stored. If the data is sorted (high correlation), BRIN is pretty efficient because we can fetch 1 MB of data, scan it, and we are done. However, what if data is shuffled? In this case, BRIN won't be able to exclude chunks of data anymore because it is very likely that something close to the overall high and the overall low is within 1 MB of data. Therefore, BRIN is mostly made for highly correlated data. In reality, correlated data is quite likely in data warehousing applications. Often data is loaded every day and therefore dates can be highly correlated.