Question
Why is the number of physical alternatives capped at 64?
Where the limit is
control_plane/src/physical/workload_cost.rs:572 (select_with_frontend) rejects more than 64 candidates:
if candidates.is_empty() || candidates.len() > 64 {
return Err(invalid("candidate inventory must contain 1..=64 alternatives"));
}control_plane/src/physical/workload_cost/materialization_candidates.rs:40 (enumerate) stops adding masks at 63, which leaves one slot for the all-Prometheus alternative that with_exact_alternative appends ("Reserve one of the selector's 64 candidate slots for native execution").
The 64 limit was added in 179c82c ("Compare complete bound workloads using strict provider cost evidence"). The 63 mask cap came in with 6f4be13 ("Execute mixed ASAP and Prometheus query DAGs"). Neither the code nor the docs explain why the number is 64.
Question
Why is the number of physical alternatives capped at 64?
Where the limit is
control_plane/src/physical/workload_cost.rs:572(select_with_frontend) rejects more than 64 candidates:control_plane/src/physical/workload_cost/materialization_candidates.rs:40(enumerate) stops adding masks at 63, which leaves one slot for the all-Prometheus alternative thatwith_exact_alternativeappends ("Reserve one of the selector's 64 candidate slots for native execution").The 64 limit was added in 179c82c ("Compare complete bound workloads using strict provider cost evidence"). The 63 mask cap came in with 6f4be13 ("Execute mixed ASAP and Prometheus query DAGs"). Neither the code nor the docs explain why the number is 64.