BRIN is 1/4570th the size of a B-tree, until 5% of rows are updated

A deep dive into PostgreSQL's BRIN index reveals a hidden cliff: updating just 5% of rows in a range-queried table causes a 28x collapse in pruning efficiency, even though pg_stats.correlation still reads a healthy 0.921. The author, a former Oracle Zonemaps co-owner, explains why BRIN degrades so sharply and offers a costly fix via CLUSTER. Includes a reproducible benchmark script.