When Is a Columnar Scan Bandwidth-Bound? A Decode-Throughput Law and Its Cross-Hardware Validation (opens in new tab)

A columnar scan that decompresses, filters, and aggregates should be limited only by memory bandwidth (the roofline floor T >= BytesRead/beta), yet real kernels are often compute-bound and leave bandwidth idle. We give a predictive answer to when a scan is bandwidth-bound. Across encodings, predicate selectivities, and two very different machines, a decoder's value throughput T_dec (values decoded per second) is essentially independent of bit-...