PromQL: topk with 'by' clause over bare instant vector fails with column resolution error

[milindsrivastava1997]

Summary

topk by (label) (k, metric) applied to a bare instant vector fails at L2→L3 conversion with a column resolution error. Related to the Aggregate(Sum) wrapping bug in bare instant vectors.

Reproduction

# P8 — topk with by, instant vector
topk by (service) (5, http_requests_total)

Error:

ERR: L2→L3 conversion failed: column resolution failed:
ColumnRef::SampleValue has no `value` column in schema (have: ["service", "sum"])

Context

topk(5, http_requests_total) (without by) produces Aggregate(Sum) instead of a plain scan. When by is added the sum aggregate's output schema no longer contains a value column, breaking SampleValue resolution.

Reproduce via example

cargo run -p asap-control-lower --example topk_ir

Labels: P8

[zzylol]

Verified fixed on latest main (via PR #31).

topk by (service) (5, http_requests_total) (P8) now lowers cleanly — no column-resolution error:

Limit { n:5, Sort { keys:[col1 desc], partition_by:[2 (service)] } → Scan }

The service label is preserved and routed to Sort.partition_by (no implicit Sum collapsing it). Reproduced via cargo run -p asap-control-lower --example topk_ir. Closing.