You have `CREATE INDEX idx ON t (a, b)` and run `SELECT b FROM t WHERE a = 5`. The plan shows `Index Scan` rather than `Index Only Scan`, even though both a and b are in the index. After a VACUUM the same query flips to `Index Only Scan`. What mechanism makes VACUUM the deciding factor?
CREATE INDEX idx ON t (a, b);
SELECT b FROM t WHERE a = 5;