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

BRIN indexes in PostgreSQL are remarkably efficient on freshly loaded, ordered data—48 KB versus a 214 MB B-tree for the same query—but degrade significantly with row updates. When just 5% of rows are updated, BRIN's performance collapses by 28x in I/O and 23x in execution time, despite pg_stats.correlation remaining at 0.921, revealing the index's dependence on physical data ordering that PostgreSQL doesn't guarantee. The author, who co-owned Oracle's similar Zonemap module, demonstrates that BRIN's advantage disappears much sooner than correlation statistics suggest.