Your result is surprising: I suspect your wide table with an index wasn't in cache, and your timestamp-only table was. The whole point is that indexes (can) pr

Your result is surprising: I suspect your wide table with an index wasn't in cache, and your timestamp-only table was. The whole point is that indexes (can) prevent full table scans, which are expensive. This is true even if the column(s) you're indexing have high cardinality: but it relies on performant table or index joins (which, if your rdbms is configured with sufficient memory, should be the case).