BRIN is 1/4570th the size of a B-tree, until 5% of rows are updated
What happened
I co-owned the Zonemaps module at Oracle, and BRIN is the same design in Postgres. It stops pruning long before pg_stats.correlation admits anything is wrong — at correlation 0.921, a 28x collapse. Measured on 10M rows, harness inline.
Summary assembled by rule from the sources below