Most exploratory questions against a large table only need a rough answer, but without an index on status, even "roughly how many shipped orders" costs a full scan. On the two-million-row orders table, Postgres has to read every single 8 kB page, all 18,085 of them (141 MB), just to count matching rows. EXPLAIN (ANALYZE, BUFFERS) SELECT count(*) FROM orders WHERE status = 'shipped'; Aggregate Buffers: shared hit=18085 -> Seq Scan on orders (actual rows=500000.00 loops=1) Filter: (status = 'shipped'::text) Rows Removed by Filter: 1500000 Execution Time: 56.188 ms shared hit=18085 is one buffer access per page, every page found...